It's not normal for most AV applications, but bitdefender is a little out of the ordinary. I have limited experience with bitdefender (I managed a pilot of it for a WinXP build at a corporation I no longer work for), but I know it's not supposed to force a reboot after every update. Iirc it should only require a reboot after the initial installation, however, it is a lot more secure in the white listing mode and there might be a setting in there somewhere that you can configure. I can conceivably see it rebooting after adding or removing entries from the list of acceptable websites and applications (which of course is updated almost daily).

The maintenance Nihil listed is always good, but I'd google around for "bitdefender reboot setting" as well.