Where does Time Machine store its cache? Mine is sometimes corrupted
There could be any number of different factors involved in why Time Machine is doing a full scan after using Windows/Bootcamp. From what you describe it seems out of sync, as if your system is telling Time Machine 'this volume is not like it was, so do a full scan'. One of the first things that might give you an idea of what is going on would be to open Console immediately after Time Machine kicks in with 'Preparing...'. By observing fseventsd, system.log, and backupd you should be able to get a better picture of what might be taking place. You can type any of those terms in the search box of Console to see a consolidated list of those events.

This article is also an excellent in-depth guide to Time Machine; it covers just about everything you could imagine, so take a look.