This occurs when at least one of the vehicles is unable to keep to its schedule due to heavy usage, resulting in it falling behind schedule and being in the location a later scheduled vehicle when that one should be there.
This occurs when at least one of the vehicles is unable to keep to its schedule and therefore ends up in the same location as one or more other vehicles of the same route at the same time. The vehicle then ends up in the location of a later scheduled vehicle when that one should be there.