It may be driven by traffic Rachel. You haven't posted on your thread recently, which probably means only a few people may have looked at it. (most traffic occurs on a collection thread when something new is posted as it rises to the top of "unread posts"). The downside may be that the more people look at your photo's, the more likely it is for them to disappear....
If you can, (and I know others are having issues) I would recommend downloading your albums from PB, even if you have them stored locally. The downloads from PB will retain the same filename as the ones used on the thread, so it makes it easier to fix in the long run.