Basically, bus bunching happens because when a bus gets behind it creates a self-reinforcing feedback loop.

Say a handful of strollers happen to get on a bus near rush hour and make loading and unloading slower than in the average situation. Now the bus is behind, and the further behind it gets, the more people are waiting at each stop, the more overcrowded it gets, the further behind it gets, and the cycle compounds until the next bus catches up and possibly jumps ahead to start picking up the excess passengers who are waiting at the stops.