Go here for some interesting speculation about the cause of a blast and fire at one of Iran's most secretive missile sites.

The most likely explanation for this is simple blundering incompetence. These facilities are underground and are used for the storage and maintenance of Iran's most advanced missile systems. Probably during maintenance some corners were cut, or some bone headed behavior lit off the fire and subsequent explosion.

My next most likely guess is a version of the Stuxnet virus, or something similar, getting into their control system and causing this kaboom, and perhaps hindering the battle against the fire.

I can't see personal sabotage as being as likely, although it isn't impossible. One would imagine that anyone allowed into this facility would be properly vetted by the Iranians for reliability, but who really knows?

Whatever the reason, this will damage Iran's ability to make mischief among it's neighbors, and that is certainly a good thing for everyone involved.