I don’t know if a complete list of triggers already exists somewhere on Meta.SE, but here it is:
- posting a question
- editing any part of a question (title, body, tags)
- posting an answer
- editing an answer
- editing a deleted answer
- applying a bounty
- reopening a question (not casting a vote, casting the final vote)
- the Community user bumping old unanswered questions randomly every once in a while
Most actions do not bump a question. That includes:
- deleting answers through the review queue (if an answer is posted and subsequently deleted without anything happening in-between, the text on the main page chages from ‘answered x ago’ to ‘modified x ago’ with the same x.) or having a mod delete them.
- undeleting an answer
- self-deleting an answer
- deleting a question (does not make it active, it simply removes the entire thing from the page altogether.)
- awarding bounties
Personally, I like using the active tab, as it allows me to see what has changed and potentially adjust my votes or recomment accordingly. The new queue only shows new questions (to the best of my knowledge; but maybe it also shows new answers to questions) and thus I am likely to miss interesting answers or edits if I rely on it.
: Recently confirmed by a post over on chemistry. Answer was deleted, then edited. Editing showed up as an active bump of the question even while the answer was deleted.
: According to pnuts in a comment.
: An answer pnuts deleted after I answered here did not make the question active again or change the active user. However, it did go from answered to modified although I did not edit my answer.