Why do I have a reputation change on my reputation page that says 'User was removed'? Includes the following:
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, resulting in no reputation change for those who had been voted on by that user.
Recently it seems we had a user removed that affected a LOT of the sites most active users. Apparently that user's rep was not high enough to trigger the above mentioned "special deletion." Yet by observing the rep changes for the Month of May, the deletion of a user put around 540 different users into negative rep territory for the entire month overnight between 5/24 and 5/25!
Although the user that was removed apparently did not have a high enough reputation, the number of vote reversals affecting over 500 users indicates it was fairly disruptive to the community as a whole.