When does an old post return to the top of the question list? It seems to involve re-editing AND a window of time, i.e., if the edit is much later than the original post, it wouldn't move back to the top, right?
Assuming that you have the active tab up - any edit (to the question or answers to it ), bounty or new answer would bump up the post. Unanswered questions are also periodically bumped up for attention automatically.
There's generally a short delay due to when the page loads as well as caching.