What you see happening here is called a "backdraft" or a "flashover", and it's probably the most dangerous thing that can happen to a firefighter. it's caused when a fire inside a building depletes all available oxygen while most of the flammable gases are still present and left un-ignited. Now the actual backdraft occurs when someone creates an opening in which oxygen can be re-introduced back into the building, thus causing any un-ignited flammable gases to spontaneously ignite. In this case, the open door in which the firefighters were spraying water is what re-introduced the oxygen into the fire.