Thats a very interesting question..hmmm. Thats my usual response when I do not have a clue.
Pure guess does it have something to do with oxygen? I mean there is more oxygen available initially...?
It doesn't ALWAYS happen, it depends on the type of combustible material that is put on top of the initial flame.........A lot of times there is an abundance of flammable material put on top of the initial flames. Such material will not burn until it has reached its temperature of flammability.
If all the material reaches that temperature at relatively the same time then it will all flame up at once........
Excluding electrical fires, why does water not always extinguish fires? The answer is, water only decreases the temperature of the burning object below the temperature required to stay inflamed. Thus, burning buildings that have built up so much heat will require a tremendous amount of water to lower the temperature below the point of flammability......It's not an issue of water blocking any oxygen.