I've seen a 55 points drop on Stack Overflow, and this is what the explanation looks like:
(There is nothing visible below the "2 events" line.)
Is this a bug?
So somebody who upvoted and accepted your answers had had their account removed, and thus their votes were removed from your posts as well. That would explain the rep loss.
I'm not sure why in the post sort it says 2 events as opposed to just 1, or why it doesn't display anything at all. Chances are that it's a bug as you have indicated in your question.