Ahem.. That EXACT same thing happened to someone else too. I think in that persons case it was the same reason, trying to modify the trap to work with pistons
I then blocked off all everything at the bottom floor and started work on the piston crushing mechanism.
It turns out that if one floor gets full, mobs start to clip through diagonals into the floor below... And it's usually a creeper.
I've used over 40 sticky pistons for a design that needs 20, because they keep getting exploded when my trap leaks creepers.
Currently my trap has holes like swiss cheese and I need to go in from the top and clean out every level to be able to fix it without getting creepers squeezed through the floor at me. :/
E2: OMG pvp is epic , got killed few times , now i founded a shelter deep down and i was lucky to find some iron right away, so i think im safe for now. xD