Why would my database revert to an earlier version

My colleague just logged in to one of our sites and found that the last 6 months worth of posts, etc. were missing.
I examined the database and saw that it had reverted to on old version. I have no idea why or how this happened. I have restored from backup but I want to understand how it happened so I can ensure it doesn't happen again.
We're running the latest WP and BuddyPress along with a variety of plugins:
- Wordpress backup to Dropbox
- YARPP
- BP Registration
- Autochimp
- HGK SMTP
- Star ratings
and some others
The theme is Mingle and we're on a shared host.
Any ideas that could help me figure this out much appreciated.