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.
Don't understand. Different songs in different keys maybe. A lot of digital artists now use pitch correction technology which could alter things. Individual tracks could be recorded at different times or locations with tuning different each time. What is the CD you are experiencing this with?
IF a 44.1k file gets played at 48k you will get that. Probably a software glitch.
Thanks for the responses. The songs themselves are not the issue because the pitch change only happened when I engaged in the "random" mode which I gathered the software somehow messed up. If I play the same song again either individually or using the "back" button when in "random" mode, it worked fine. So definitely a software glitch but it's weird that different tracks would either play at higher or lower pitch. Maybe I should check to see if the affected songs were encoded at different bit rate thus the varying response.
It is not a bit rate issue but rather a sampling frequencey issue.