I've got a fairly basic spawner setup, using an MFR auto-spawner and a TCon/autonomous activator setup for killing the mobs (see video here).

It was working for a a few days killing witches, but I decided I wanted to kill blue slimes instead, and I widened the drop hole for 1x1 to 3x3. It seemed to work, and I logged out for the night. When I logged in the next day, I found...well, this:

http://imgur.com/a/6vvUZ

In short, the kill box has been blasted to pieces, and a rough tunnel has been dug straight down below it.

Does anyone have any ideas what could have caused it? Even if it was spawning creepers instead of slimes (and it wasn't), nothing should have exploded! Do mobs actually try and attack the autonomous activator? I've got other autonomous grinders running, and I've noticed nothing of the sort.

I can rebuild this, but I'm reluctant to do so with no idea what went wrong!

(Is it possible this was caused by slime jockeys? But again, why would they explode when there are no players even logged into the server?)