The "bump" button which you can see on the right side of your topic's original post overrides the last's post date so when people use that, they'll end up being on the front page, even if they haven't posted anything for some time.
For additional information, check this thread
Bumping changes on some boards
What you need to know
1. Don't worry about it too much. If you behave as normal, you'll do OK.
2. If you like a topic and you see a "bump" link near the "watch", "print", etc. links in a topic, click it.
3. If you've been spam-bumping, knock it off, since it's not going to work anymore.
Thanks Now understand bumping properly...