My Ignition Coil Exploded?

My 1984 325E BMW wasn't starting. I tried all the basic tricks to get it going, jumping it, ensuring snug connections on all wires, making sure the battery was fully charged. After some investigating, I believed the problem was with my Ignition coil, so I replaced that. The battery was weak, so I used a portable jumping battery to try to get the car started. The engine cranked, but wouldn't turn over. So figuring this problem was beyond my skills, I had it taken to the local repair shop. The next day they called me to inform me that my ignition coil had exploded. I went up there, and to my chagrin, it had. There was black goo all over the engine and on the windshield. They claimed that they had only charged the battery and tried to jump it. I have my doubts, cause that's exactly what I had done to it. Anybody know what would cause an ignition coil to explode?