I wonder what happened that caused the chimps to become aggressive like that.
They are naturally. Male chimps are known to group together, psyche themselves up by grooming each other, and then go out to find a male intruder on their territory and beat the hell out of him, which normally involves ripping off his testicles (an evolved tactic to prevent enemies from reproducing). They're naturally mean sumbitches, just like us. When humans enter the picture and invade their territory, what do we expect them to do?
If only humans were less like chimps and more like bonobos. Life would be sweet for all.
We are more like bonobos than chimps, generally (if we're splitting the species of bonobos and chimps and simply grouping them under the same genus, which I don't agree with... I'd rather call them variations). They're more monogomous than their cousins, but just as aggressive. In fact, whereas chimps often live in a very stratified yet stable environment consisting of many pairs of mates, bonobos will attack any intruder on their immediate family's turf (the males will, anyways).