A user who voted for one or more 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; you won't be shown which specific posts were affected.1
This also occurs when a user who was the final user to approve one or more of your suggested edits was deleted, which causes the reputation gain from those suggested edits to be reversed. Source
The reputation adjustment always occurs whenever a user is deleted, unless that user has cast a very large number of votes. In which case, deletion will be delayed so staff may consider preserving the votes prior to the deletion. The decision is at the staff's discretion and cannot be reversed after the deletion has taken place. If you are seeing a "User was removed" event in your reputation history, it implies that the user either hadn't cast enough votes to be reviewed, or staff made the explicit decision not to preserve the votes. (source 1, source 2, source 3, source 4)
The behaviour of deleting votes has been discussed in the question "Don't throw away all votes when a user is deleted".
1 At one point the events were broken out, as shown at Strange "unupvote" pattern across multiple users.