I thought it was spread by Red Templars not naturally.
In Emprise, it was, but there were veins that were actually simply discovered in the Hinterlands by miners. The various Carta notes in Valammar talk about it. Varric also mentions something along the lines of "Poor sod didn't know what he was dealing with." when you see a dead dwarven miner corpse near one of the destructible red lyrium nodes in the cave directly in the south east corner of the map. Dwarf Son Valley, I think? Anyway, this was before the Red Templars are actually a thing.
Well, the red lyrium had to be specifically grown from living hosts, didn't it? If so, the deposits might be restricted to the surface... though given the way it grew in IHW, I kinda doubt that, it must be a serious horror show down there.
That's what I'm afraid of. It did have to be grown from living hosts. And the spread started when the Breach happened.
I suppose it could be resulting from... darkspawn dying in droves to fade rift demons down there and bleeding on lyrium? lol
Or maybe some of the red lyrium we can see in the Fade leaked out into the deep roads and just started growing and the surface stuff is all branches from the same deposit? I mean, before this there was always blue lyrium in the Fade. The Fade and the lyrium underground have always had some kind of connection we've never had elaborated on much.
Or perhaps somebody who can spread the Blight, which in theory was contained in the Fade at the some point with the Black City, used the weakening of the veil caused by the Breach to leave the Fade and is down there wreaking havoc and infecting blue lyrium and/or killing people/darkspawn?
No, I don't think so - it only makes Red Lyrium grow and spread faster. I mean, if RL was only grown out of people, how come so much of it under the Temple Of Sacred Ashes?
I have a theory on that. We know there was lots of regular blue lyrium in the mountains around/under the temple. There were Grey Wardens with Corypheus, who are blighted (obviously). So: Explosion goes boom -> Grey Wardens go boom -> at least some of them go splat against some of the surrounding lyrium-> lyrium gets tainted.