Wait, didn't Tevinter have gods on its side, while all the elven gods had vanished? It may be as simple as that, depending on how much of a direct hand the Old Gods could have taken... hell, maybe they can mentally command dragons and ordered huge flocks of them to burn the elven empire down, or something (which, come to think of it, happened on a smaller scale in Dawn of the Seeker).
Well, I really don't see an army of dragons helping Tevinter, and the Old Gods were imprisoned, but it's true that WoT points out the help the Tevinters were given by the Old Gods. They are credited with teaching magic to the Dreamers of the Neromenians, thus shifting the power in the tribes to those mages (-2800 Ancient) and it's said that the first known person to use blood magic was taught by Dumat (-1595 Ancient).
It wouldn't far-fetched to think that the Old Gods were interested in the fall of Arlathan. Maybe the elves were taught magic by the Old Gods too, but weren't useful enough for them. Then came the humans and the Old Gods might have seen them as the perfect puppets.
Seeing as we have no evidence that any of these gods exist, any kind of theory based on the involvement of gods is out of left field. At any rate, we know that imprisoned Old Gods can't do much of anything; if they could, we'd presumably see them to do things in the current DA games, since there are still existing OGs.
No evidence? We killed the corrupted Urthemiel in DA:O. They did exist. However, if they can be considered gods or just especially intelligent high dragons is another matter.