We know that the elves settled the Dales sometime around 1025 - 1040 Tevinter. The Dales, were located between Orzammar, the Avvar and the Kingdom of the Ciraine (which later became Orlais) which had been a principal power behind Maferath's host (and it's first king was one of Maferath's sons).
The elves thus chose their new homeland between two existing human populations and an virtually all-important trade route. It was probably reasonably empty lands and I doubt any humans were displaced. But it was rather close. Halamshiral itself even lies on the imperial highway. So contact with humanity must've been nigh unavoidable.
During the same period various Andraste cults are cropping up and the Chant of Light itself proves to become very popular in the Kingdom of the Ciraine. It is a proselysing faith but also hunted by the old imperial faith, so it's probably as close knit and relies on the social groups surrounding the revered mothers.
We know the elves "rediscover" and "reconstructs" their old faith. How they do so after centuries of slavery is beyond me, but I wouldn't be surprised if they borrowed heavily from the faith in the old gods (that's not saying they believe in the old gods or that it's not the same gods they worshipped in Arlathan). At some point the notion to restore their immortality arrives.
Here comes the problem. Their immortality has been lost since contact with humans started. Logic suggests that contact with humanity is at fault. So two generations or three in, it's not unfeasible that a isolationist party starts rising.
The Dales lie smack in the middle of two large human populations. Right on top of the imperial Highway. Between the centre of the proselysing faith and both pagan humans and the homeland of the prophetess.
Isolation will be practically impossible to maintain.
It's not unlikely that the elves institute the emerald knights to maintain their isolation roughly at the same time Kordilius Drakon begins his wars of conquest. So sometime mid- or late- 1100 Tevinter.
Let's point one thing out: The elves institute an order of warriors to keep humans out. People trained to kill to maintain territorial "purity".
The Dales are huge. It's probably easy to cross the border by mistake. I could easily imagine a year passing before they find you. Hunters, peasants looking for land, vagrants, explorers... all entering the Dales more or less intentionally. And traders seeking to travel to Orzammar or what will be Ferelden to trade passing too close to Halamshiral. All rebuffed, with more or less hostility.
There's probably no killing, but I can't imagine the local humans being too keen on our angry elves in green.
There's probably missionaries too, some headed towards the elves some just taking a shortcut. They too will be rebuffed.
But regardless of how violent the elves are at this point, the movement of the humans into the Dales is unlikely to cease. As the wars rage and Orlais forms, chances are it just keeps increasing. At the same time, the fereldans are converting on the other side of the Frostbacks and trying to establish contact with their fellows in faith in Orlais and the dwarves close the deeproads and become dependant on surface trade. And the Dales are absolutely massive, today they make up a solid third or quarter of Orlais. It's probably not unfertile too, which makes it very attractive land.
Cue frustration on the elven side.
Bam. Chant of Light is established as state religion in Orlais. The Divine is elected. The Dales are between 160 and 175 years old. The elves who fought with Andraste are probably long dead. The humans who fought with Shartan definantely are (by my counts this is their great-great-great-grandchildren).
Most contact elves and humans have had for 4 generations are at this point probably more negative than positive. All they can remember are bad things and not even their elders will remember much good (if the elves have elders that have lived that long, they probably think humanity has taken a turn from the worse).
The year is now 1:1.
The political situation in Orlais leads to magic being outlawed and the Inqusition is turned into the templars (it was probably a good idea at the time). It's unlikely that the elven mages and the inquisition got along, the sentiment probably lingered in the templars. Unless the emerald knights had mages among their lines it's unlikely to lead to outright conflicts yet. A year or two later the templars will probably be sent with missionaries. They'll probably still be rebuffed, but I imagine it just enough threat of force to "allow" missionaries travelling the road (again, past Halamshiral).
1:5 the 2nd Blight begins. It will continue for 90 years. Orlais is ravaged hard. In 1:25 the elves refuse (despite being able to) to assist Montsimmard, what probably is the empire's second largest city. Don't think it's a stretch to say relations are at a new bottom. In 33 the Grey Warden's convert to the Chant of Light, spreading orlesian Chant of Light to the north over the next century.
In 1:45, Drakon dies of old age. Having been a ruler for 55 years (so he must've been 60-75 or older). Kordillius II is not his father.
1:65 Anderfels secedes from Orlais. Kordilius II is still the monarch, his position is probably rather tenous. Between 45 and this year both Free Marches and Antiva turn Andrastian thanks largely to the Grey Wardens.
1:95. Zezekiel dies, the blight ends. 90 years it took. During it's course half the world has turned Andrastian and Kordillius has lost half of Orlais. If he is still alive he must've ruled for 50 years... which suggests that he is not (since he wasn't much of a ruler). It's probably a son, grandson, usurper or son of a usurper ruling. Either case, they probably need a triumph.
The next five years are known as the Rebuilding. Trade, culture and Religion flourishes. Due to the Blight, the elves have probably had a great respite from humanity's encroachments. Since they didn't participate the emerald knights are still strong. Now all that encroachment resumes and with greater vigor. By now the Andrastians probably feel it's their duty to educate the pagan elves.
10 years later in 2:05, 80 years after Montsimmard (short enough for grandparents to remember the elven betrayal), skirmishes happens. We're probably talking templars violently defending missionaries, elves burning down encroashing farmsteads and chasing of the peasants, hunters being punched out. One sides does something, the other side retaliates somewhere else.
A cycle of violence. It's probably more fistfights and waving of swords and less outright fights. People do get killed, but more by accident than intent.
In 2:09 Red Crossing burns. The elves burn a settlement on the elven side of the border, the priestesses are all killed. Orlais declares war.
In 2:10 have captured Montsimmard (is Lydes and Verchiel elven or orlesian?) and are entering the Heartlands. I don't think we can expect the priests in Montsimmard to have fared much better than in Red Crossing. Val Firmin, Val Foret, and Val Royeux are all threatened.
So... one year and they're near Orlais heart... the elves were ready for this. Or the orlesians very unprepared.
The Exalted March is called.
Somewhere between 2:10 and 2:20 Val Royeux is sacked.
2:20 Halamshiral is conquered, the Dales fall. The elven communities are uprooted, probably since the Dales are huge and there's no cities. It's a recipe for neverceasing guerilla warfare to let them stay. Their faith is forbidden. Partly because it's a pagan faith, partly because it's attached to the elven-immortality philosophy that's very hostile to humanity.
The war took 11 years and led to the destruction of two capitals and at least another prominent orlesian city. Orlais alone could not have survived and it called upon the rest of Andrastian thedas to help.
To summarise. I think the fall of the Dales is a result of Elven Isolationism and orlesian expansion and how trade, hunting and religion brought those two into conflict. Even if the Orlesians hadn't expanded in that direction (something that wasn't planned and organised) sooner or later the elves would have to expand either into Orlais or into the Frostbacks. The elven position was simply unattainable. The fall of the Dales was simply unavoidable.
That's my analysis of the lore anyways. Hope you enjoyed reading it.