After looking through the sunbrand quest dialouge via the wiki, I have my own theory behind ivan soulsinger's betrayal.
According to Sunbrand, Ivan has always been a friend to the drakes, and once fed him some ambrosia. He also was apparently a student/resident in dragonspyre academy, since the charm bracelet you get from giant homunculus is from that place, according to sunbrand, as it has ivan's name etched/engraved on it.
When you go to investigate ivan at flamewind tower, he says the following: "The flesh is weak, young Wizard! But don't worry! When I've taken residence in your body, I'll take good care of it. I'll even come to visit you now and again! Ha ha ha!".
After reading about that, it got me thinking. I have a feeling that what sunbrand said is true and that ivan was always a friend of the drakes, willing to help them out when he could, but when the dragon titan invaded, ivan was possibly possesed by one of her minions, and thus turned evil and enslaved the drakes along with avalanche and giant homunculus.
Again, this is all just going on the quest dialouge that sunbrand says during his quest line, so maybe in the future there can be a quest line/dungeon that lets us go back in time to dragonspyre academy and see what really happened to ivan and the drakes during the titan army invasion.
Well there you go, my theory on ivan soulsinger and his "betrayal" of the drakes. Please tell me in the comments section below what you think of this.