Had the gum grown over the root surface at all before you tried to cement it back in? If it has split in a big way you would probably be able to see it move when the crown was out.
Sonic - its not necessarily decay that causes crowns to fall out. If the seal of the cement is broken then it will fail...simple. That can be caused by a lever type effect for instance on a front tooth - or as possibly the case in Pete's the root may be split.