Like Pisces said, you could have been too close or something. It happened to me once. I was finishing a ship off from ~150m with the flak gun, (out of torpedos in a IIA). It looked like it where about to go down, but I had nothing else to do so...
Suddenly it blew up in my face and I heard my hull crack right open. It took about one or two seconds, (maybe three, with the reaction time in mind) until it was all over.
I was stunned and regretful.
