I don't really see the point in bumping a question to the top of the "All Activity" page when a question is retagged. There's really not many malicious things that you could do by retagging a question, and even if someone did, for whatever reason, retag something to be offensive, it would either be quickly noticed, if it's a recent question or a question with recent activity on it, or not noticed at all, if it's an old question with no recent activity on it. Retags would be useful for getting rid of old tags with only a few uses without pushing everything out of the "All Activity" page. Also, as a side question, I know that mods can silently edit posts so that they aren't bumped to the top of the activity page, but does the same apply for retags?
PX Note for users: If you find something you think needs to be retagged, just link it to one of our profiles and we'll deal with it.