'Answer removed' will appear automatically if the poster is later banned (as all of their posts are removed from the site).
It will also appear if the Ed or a moderator decided that an individual post was offensive or otherwise inappropriate.
Further, some moderators (including me) like to 'tidy up' threads after a post has been removed. For example, member A might post something offensive, resulting in members B, C and D immediately responding with posts such as "That's disgusting!", "Grow up, you idiot!" and so on. When I remove the post from A, I also remove the responses from B, C & D as well, as they no longer make any sense in the absence of the post which spawned them.