It's rare to happen, it's more like on privacy reasons to the extent it only happens to some users who wants to quit here and forget their usernames.
Well, some active users have very good reasons for theymos accepting their request. I once requested before but failed lol.
Actually I won't consider it as a rare occasion especially from the fact that some members of the forum were granted last year till now, the best criteria for picking is base on The Sceptical Chymist reply