Attacking the water pump

Lots of possibilities. Resource denial. The biters may have enough of their own water and they do not need the extra. Attack it to deny it to the colony. Scarcity of resources. Say lots of the biters live underground. They might have access to the aquifer from their own tunnels. If the humans are now trying to steal it with their well, maybe it is time to put a stop to it. Maybe they did plan to try and use it for themselves, but king Datu kicked their ass so they fled.