I don't known why those particular users deleted their accounts, but yes, that is (usually) what a grayed-out username and "blank" avatar indicate. People can leave for all sorts of reasons; they got into an argument with a different user, they don't want their name associated with the site, etc. There isn't really much we can do about it; the moderators can ask the user to reconsider, but if they confirm they want it deleted, we'll do it.
When a question is migrated from one SE site to another, and someone who posted an answer on the source site does not have an account on the target site, then after the migration, their actual username is displayed, but grayed-out, and with the "blank" avatar (if they later extend their SE account to one on the target site, the username and avatar should update to normal). Some people have usernames that are of the form "user####" in which case this appears the same as the previous reason (though checking through the users on the source site will reveal if there is someone with that username there.)