It seems like simply changing the tags for a question shouldn't cause it to be active for three reasons.
- There isn't really any activity on the question. Some metadata that I suspect most people don't even read changed, but nothing important. (I realize some people think tags are of supreme importance, but hopefully you'll agree that they're not integral to asking and answering questions.)
- It allows gaming the SE system whereby a poster can mistag (or add nonsense tags to) a question and thus potentially have the question reappear at the top of the active list once someone retags it.
- It is really annoying to see a huge list of old questions scroll the new questions off simply because they were retagged, as just happened.