You thread not disappear... still there... even if BUMP, it will appear for only a few minutes in 1st page because everybody else also BUMP there thread after you. That time your thread move down. Not mention if others thread got new reply, that thread will move up. New thread also will place up...

In section where got so many active user, some inactive threads will get buried really fast.

This is when your thread last active.
1st - Today, 10:50 AM
2nd - Today, 11:23 AM