There's another possible cause, based on where the outer chainplate appears to have failed: the broken plate could have been weakened by the pin being forced in at a less-than-perfectly-orthogonal angle when the chain was being put back together. I've certainly had trouble when trying to reassemble a chain with a cheap or worn-out chain, as evidenced by seeing the back-side outer plate buckle rather than pop onto the end of the pin.
Just a random guess, though. Regardless, I'm glad you avoided any serious injury when it failed.
