Losing cache / re-generation of thumbnails cause?

So, I'm not sure if this is a bug.
here's the problem: Sometimes when I reopen Bridge, a gallery I'm working in that was previously cached in full suddenly decides it needs to regenerate all thumbnails, which can take a while (I have it create monitor sized previews). I usually work from either a local NAS or thumb drive.

What can cause this??

"Compact cache on exit" is UNchecked. There's always room to spare in total cache too (just now it happened; cache was under 13gb and allowable total cache was 260gb). 
this has been a problem for me on multiple laptops/desktops for at least a few years, but I only notice it happen once every couple months.