Possibly YOUR IPOD looks for the matching songs/vids/etc. on THEIR PC, but doesn't find them, and so it says, "these songs must be corrupt, even though I have a copy of them on me. I'm just going to delete basically everything for no valid reason at all."
And vice versa (THEIR IPOD, YOUR PC). I have no idea why that happens...whoever coded that may or may not have been paying attention.
/ 
