Currently, editing a question's tags causes the question to be bumped to the top of the active tab. As I understand it, a question being bumped to the top of active is saying "Hey, community, something interesting/substantial happened to this question and you might want to check it out!". In my opinion, tag edits are not substantial enough to make it worth bumping the question, as they are mostly janitorial edits.
Consider the following common reasons for editing a question's tags:
- The question was tagged incorrectly, by using an irrelevant tag or a combination of tags that don't make sense together
- A tag that the question used has been retired or otherwise changed so that it no longer applies
- A new tag has been created that would apply to the question, perhaps better than one or more of the current tags
All of these are janitorial edits - not substantial enough to warrant increased attention from the community, but rather something that needs to be done to keep the site clean. In the real world, it would be like getting an email at work every time the maintenance staff replaces a light bulb.