It sounds like you had a momentary ignition failure - - - -maybe an open or short in the wire between the coil and the distributor, or a wire inside the distributor getting shorted for a short time. It's the equivalent of turning off the ignition switch for a short time, allowing unburned fuel to accumulate in the exhaust, and then turning the switch back on. The boom and flame out of the tailpipe can be very impressive!
Try jiggling the wire between the coil and the side terminal of the distributor to see if you can make the engine stall. Since the vacuum advance keeps that connection in constant motion, it's not uncommon for the strands of wire to get fatigued and develop an intermittent open circuit. I like to use a piece of high-flex wire that's normally used for test equipment leads in that application.
Jerry
