The apes territory and resources have been taken from them by an intruding clan and when it dawns upon them what the monolith actually is they suddenly gain enlightenment. This leap of intelligence enables them to take back their territory. Notice that before the fight they are seen outside their cave with the sun peaking over a hill on the horizon again. They are planning and preparing their attack. They are the enlightened base of the pyramid and like the morning sun are about to rise and enlighten the landscape.