A user who voted for one of your posts had their account deleted (either by request or due to violating the network's terms of service). As a result, all of their votes were removed, and the reputation you gained or lost from them was undone. The resultant reputation change could be any amount; it could even be a reputation gain if enough of the removed votes were downvotes. All the removed events are rolled into one, so there is no way for you to find each affected post1.
This removal occurs whenever a user is deleted, unless that user had a very high reputation score. Because high-reputation users have usually cast a great many votes, removing all of them could be that much more disruptive to other users. In such cases, the staff use a special deletion that preserves the votes (source 1, source 2).
The behaviour of deleting votes has been discussed in the following question:
The message displayed in the reputation history has been discussed in the following questions:
1 At one point the events were broken out, as shown at Strange "unupvote" pattern across multiple users.