Why unused files become fragmented?

I have question about fragmentation in general. I know how fragmentation works and why it happens, but I can't explain this situation:

I have partition where I store all my movies, music, and nothing else. So most of the files are about 700MB+. I'm using Defragger, and it says I have 0% fragmentation on mentioned partition. Good so far. Yet, after few days some of these 700MB+ files become fragmented, even when I didn't open these files. It looks like the files are fragmenting itself without any reason. Or maybe there's something that I don't know?

My question is: why this happen? And what to do to prevent this unreasonable fragmentation.

Hi, I don't know off hand what could be causing it but if there are red blocks showing, click them and see what they are; that might give you some clues.

I was also thinking it might be Windows Indexing Service or Google Desktop, if you have either of those enabled, but I don't think they do anything that would cause fragging, but I'm no expert so it might be worth looking into.

Good luck. :)

After defragmentation, there's no red blocks. Yet, few days later they appear, even if the files that are now in red blocks wasn't used since defragmentation.

I don't use Google Desktop, but I follow your advice and turned Windows Indexing Service off. We'll se If that helps.