It seems to be the case that for files which hold a local copy of their Mailbox are prone to corruption.
The causes are unclear, but bad internet connection, errors caused by Microsoft automatic updates being installed on machines with Outlook still running at the time of shutdown and general syncing errors between the remote mail server and the client are all suspects. This seems to be a particular problem for Outlook 2013.
If your Outlook does start to behave strangely, with trouble selecting emails and obviously wrong information being displayed, here’s a Microsoft article which show how to fix it: