I dont have a scientific response to give you but that sometimes happens rather randomly... its like sometimes when you put a pen drive in your computer and it asks to format it just because lol... i think it might have to do with the operating system doing something it shouldnt when taking the drive out the last time you used it or a hardware bug, but usually when it happens i just end up formating it since it doesnt have anything important most of the times. Though i never heard of that happening to memory cards i supose its about the same, if you dont have important pictures in it just format it (Remember that everything goes Caput) but if you, do you should try to use some low level recovery system to try to get anything out of it before you format it...
PS: i think the problem here is a bit of information that the memorycards have thats kinda only there for the machines to read about what it has on it, and it might have gotten corrupted since you used it last... with a bit of luck if you put the card on another camera or a card reader maibe it can salvage some of this information and do a kind of recovery and repair that information without the need for a format =)