Psh. Pandas don't need to be aggressive. Everyone knows not to F with them, so everyone backs off.
It's like Mr. Miyagi, you know? He kicks ass so much, and you know he does, but he doesn't NEED to kick ass. Remember that scene where that asshole totally fronts all up on him and he's like, super calm. And Ralph Machio is like WOAH SLOW DOWN WHY DIDN'T YOU KICK HIS ASS and Miyagi's just like... man.... you don't need to do that shit to make your point.
That's pandas, man.
TomFulp
A holographic recording from an Alliance officer explains that the Alliance attempted to bring peace to the population by filling the atmosphere with a drug designed to suppress aggression. The drug instead suppressed the pandas' motivation to do anything, and they died of starvation, but 0.1% had the opposite reaction and became the hyper-violent kodiaks.