Clumping can be caused by random heavy usage of any particular vehicle, resulting in it falling behind schedule. The leading vehicle eventually lapses towards the time slot of a later scheduled vehicle. Sometimes, the later scheduled vehicle gets ahead of its own timetable, and the two vehicles meet between their scheduled times. Sometimes one scheduled vehicle may pass another. These effects tend to accumulate, unless the later vehicle deliberately idles on stops or slows down, or the lead vehicle skips some stops where it has no drop-offs, leaving the pick-ups for the following bus.