Jason:
It is called an explosion. In the coking process, the gasses produced are extremely flammible and much of them goes unburned. Unburrned flammable gases collected someplace probably in the vent pipe and when they found their way back to the fire and ignited with explosive results. Glad you were not injured. I once left my rivet forge for lunch, put a block of wood in the fire and covered it with green coal and patted it down with a shovel. When I finished lunch, I came back fished the block of wood out of the fire, ignored the smoke wafting out of the intake for the blower. There was a boom that rattled the windows in the house, caused me to jump clear over the anvil and the neighbor lady to come out and see if I was ok. The unburned gases from coal are not only very flammable, then also contain numerous toxic components.
Woody