I'm fairly new to SE, but I've noticed a lot of questions that are "bumped" back to the top of the list, yet when I view the question there's no visible activity with a recent timestamp. In some cases, the question, answers, & comments are all from 2011 or 2012.
Are answers being bumped because someone up-voted an answer or comment? Or do questions only get bumped when an answer, comment, or other edit is added?