Endless is very right. Alot of the time, Trance is "plot related". I haven't played FF9 in almost 2 years but from what I remeber, characters get their trance due to people dying, importance of battle or just emotional battles.

- Very start where Dagger is trapped in that leaf monster, Zidane uses trance to free him.
-Vivi uses trance when he fights Black waltz 3. (famous image of him casting fire)
-Quinas fat

Sometimes you get Trance on literally the last hit of the battle and sometimes you get it at the start of a boss battle. It's all about luck. (but as endless said, you can predmediate boss battles)