Why do my characters randomly learn new Overdrives?

[Peter_20]

They are called "Overdrive Modes". They are the way you can fill up your overdrive gauge. If my memory don't fails, you can change them in the character's stats.

For example, if you equip Yuna with Healer, her overdrive gauge will fill up everytime you heal someone with Yuna. There are other ones, if the character get hurt, if the character kill a monster, or even if the character is the only one standing. The ones you get depends on how you use each character.

Wow, sounds cool.

In that case, I'll have Tidus on Slayer Warrior while Yuna is set on Healer.

...because I always have Tidus kill attack monsters and Yuna heal.