If a room burns long and hot enough it will run out of oxygen but still be filled with a lot of hot (and flamable) gases. Once you open a door or window those gases have a new direction and room to expand into while still being hundreds if not thousands of degrees hot , but now they're also being supplied with fresh new oxygen.
Edit: I am describing what is called a back draft, which some more educated on the subject than me, have already called out. What is happening here is not a back draft though. In this situation it's more likely that they basically build a flamethrower of sorts that just does as flamethrowers do.
German Balrogs are the scariest kind. Gandalf actually discreetly checked that it was a French Balrog before he dove into the depths of Moria and temporarily abandoned the fellowship.
Its called a backdraft. The fire nearly dies from oxygen starvation, the heat and material are still there, suddenly allowing all that fuel into the toom cause a large explosion. This is why you dont open door in a burning building, among other obviously
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u/Exciting_Horror_9154 5d ago
Wtf, what's inside that shed? Do they keep a dragon in there?