Pitch changes with digital music???

I was using Plex (beta software) to manage my digital music collection. Recently, when I was playing some music on "Random" mode, I guess the system was running out of juice. In any case, what happened was some tracks suddenly play at higher pitch and the next track lower pitch (i.e., like a record with the wrong rpm). My question, mainly just out of curiosity, is how can it happen with a digital file? Thanks for your input.