Pretty simple bug here. Thrash is a move that lasts 2-3 turns randomly, consuming 1 PP for the activation of the move. Pixelmon currently consumes 1 PP for each turn of the Thrash, which is not consistent with the mechanics of the move in the Pokemon titles.

Example: In Pixelmon, my Braviary uses Thrash and continues the attack for two additional turns, for a total of three turns. 3 PP will be consumed, one for each turn of Thrash being in effect. In the Pokemon titles, my Braviary uses Thrash and continues the attack for two additional turns. 1 PP is consumed upon execution of the move, but not for subsequent Thrash attacks in the same chain.

I have tested this with Outrage as well, and it exhibits the same bug, and although I've not tested this with moves like Petal Dance, I would be willing to wager it works the same way.

Tried it and it works just fine.