Did he continue to cast on you after that and noone else? No? Then its random aggro. "Random aggro" means that he does not follow a regular hate list mechanics, which can even be caused simply by his hate list resetting too often. Perhaps you hit him at just the right time. And also, one instance of him turning on you is hardly proof of anything. Repeat it several times with the same effect, then we can talk.
Fireball is an AoE spell. Stand near healers = healers (and you) will get hit by it whether it is you being targetted or the healers, effectively doubling the chance of "getting a fireball to the face just because" for both you and the healers.
EDIT: Also, Id love to see the 274 HPs breakdown, because the only way I can figure out a lvl17 pure sorc could have exactly that number would be 26 total con, toughness item, GFL item, no toughness feat/enhancements. You said you werent sure in the OP, but the way you put the new post seemed like you already confirmed the number, so just curious.