Imroved Trip and AoOs

[Liquidsabre]

The answer you receive is "Oh, spells are different."

No more different than a grapple attempt. AoO's occur before the action that triggered them is completed, and only interrupt that action if there are special rules that state that they do so. As with a grapples, so is it for spells, only for spells the caster has a chance to ingore the damage and complete their spell - the grapple attempt fails if the character takes damage from the AoO.

It's really just a rule-of-interruption for special cases involving AoO's.