Why do some MIDI files insert numerous Channel Volume changes for some notes?

I can understand a need to change volume when the score requires it, e.g. going from a passage marked Pianissimo to one marked Forté etc, but when there are no markings, why have the volume wnadering up and down over say +/- 5 or more and maybe 20 times per measure/bar?

I'm finding Midiocre works well for MIDI editing.