The problem is that it lets you perform a trip attempt as a consequence of dealing damage, not directly tripping them as a result of doing damage.
Being rendered prone is a result of the trip, and being tripped is a result of the damage, so being rendered prone is a result of the damage, albeit indirectly. I don't think we disagree on this exactly, I just want to emphasize that an indirect result is a
kind of result.
I have already said that whether dropping a creature can be an indirect result of the damage is not clear from the wording. If you feel this restriction can be inferred, you should make an actual argument to that effect.
While we're on the subject, would dieing because you failed the fortitude save you have to make after taking 50 or more points of damage from a single hit be a direct or indirect result of the damage? I think indirect, since there's a check made between taking the damage and applying the effect.
the only thing that also might trigger it (stretching what i would allow) ... would be an attack that did
So you're thinking no on the massive damage thing? Are you saying that because it's indirect?
As a free action you can Drop Prone.
So, dropping is a way someone can become prone. This doesn't prove that dropping is the only way to become prone. Without that, you can't say that, just because someone is prone, he must have dropped.