It happens when an unruly post from somewhere else wanders into this thread and rudely knocks your post over.
Kinda like the old Batman show where you saw Kapow! after the Riddler landed a much deserved punch in Batman's ever tumescent belly...
Or it's that your original post has been pushed down the list by others, and you still have that shiny, rust free tail light that somebody really should buy, so you can get a cold beer or two with the cash.
So you reply with "bump" and push it to the top of the stack. Usually accompanied by a reduction in price...
Cheers!
Awesome price, and it would be a good partout if the motor ended up being toast.