In an airtight room when there is a fire it consumes the oxygen and becomes ventilation controlled the room gets hotter and hotter and the combustible solids in the room continue to pyrolyze into flammable gasses but can’t burn because there is no oxygen. Then a firefighter opens the door…

There’s actually a vacuum in the room, it sucks fresh air in and all those flammable gasses and smoke ignite and explode.

It’s called a backdraft. It only happens when there is a tightly sealed home/room.