Broken stero or ?????

I just wanted to ask a question...wondering if anyone has any ideas...

Today I was driving our fairly new car to work and just as the chorus to a song started the volume on the stereo went up. I thought it was weird and turned it back down. It happened again at the start of the next chorus.....and the next (each time I turned it back down). All up this happened 4/5 chorus' (from three songs in a row) and then stopped. At first I thought it was just due to going over a bump or something....but for it to happen at the start of chorus'???? Seems like a weird coincidence to me.... What do you think??

EDIT - and yep, it appears I can't spell in my title....