User mysteriously deleted
When a user is deleted from the system via the Craft control panel, they are presented with a model offering them the choice of what they want to do with that user's content.

The options are to 1) transfer it to a different user (which you can select) or 2) delete it entirely.

So the behavior you're describing certainly sounds like someone went through the UI to delete a user and transfer their content.

You might want to consider installing something like the Audit Log plugin to catch the borderline criminal.

The only other time we've seen a user and their content get deleted is from a more convoluted process where a Craft install has users in a "pending" state and they've never authenticated their accounts and they start assigning content to that account (via a custom plugin or some such).  They also have the purgePendingUserDuration config setting set to something other than false (say 1 month), then once the user account has been "pending" for a month, they get deleted along with all of their content.

But it doesn't sound like that's what is happening for you.