I'm not convinced it was either the mages or the templars that caused the blast. Seems to me there's something bigger manipulating the factions. I doubt the wardens would be an enemy faction if this was just about mages or templars sabotaging the summit. My guess is, someone else did this and caused the breach. Maybe Corypheus, maybe Flemeth/Morrigan, or maybe even Fen'Harel. If none of the above, probably another major power that we don't know about yet. I doubt it's going to be a direct cause of the mage/templar war, though. It seems to me that someone is just vested in causing chaos everywhere.
I like this.
Me, I'm more inclined to see it as an unknown third party. Mages don't seek to destroy the world, they seek to live in it without being seen as monsters to be tolerated, and templars are more concerned with the safety of the majority, not just the mages, and for the most part want mages isolated and sealed away to contain and minimize the potential damage.
Neither side has any real motivation to breach the veil and cause a hailstorm of demons to rain on the earth.
I can see Tevinter or the Qunari doing it as a way to sow chaos and make the southlands easier to conquer, but the Qunari are terrified of magic, seeing it as a contagious disease and Tevinter is too involved in its war with the Qunari.
I suspect Flemeth or an otherwise unknown third party at this time.