Have you ever been waiting for a late bus, and all of a sudden two or three buses arrive at the same time? That's called bus bunching.
Bus bunching happens because, if a bus gets delayed, then there will be more people waiting at the next stop than anticipated. The extra passengers' boarding time makes the bus even later, and so on in a vicious cycle.
It can be easily understood on this website: http://setosa.io/bus/