It is used on old posts just to give them a more current date, in an attempt to bring attention to the post's contents. It implies that other people (the "bumpers") are looking for information on the same topic, and can also be thought of as a second, as in "I second the motion", but more like "I am also interested in an answer to this question/issue", particularly when people are bumping when the post is NOT old. The word itself is used because the comment/topic is getting "bumped up" to the top of the list within a forum.