It has to do with the quantity of combustable fuel available. Many fuels are more combustable when they are hot and are off-gassing. However, while these are more common in modern usage, they are not the typical case.
Try lighting a candle ... no huge flare. Light leaves, an old oil lantern, paper, charcoal with wooden matches.
If you have an explosive mixture of fuel (propane BBQ grill, lighter fluid, phospohorus - well, then things are a little more interesting, aren’t they?
Ok but given a presumably abundant quantity of combustible fuel that feeds the "excess" -- that is, the flare -- what was this quantity doing in the previous second? Why did it not ignite exactly when it was at the quantity, or the density, that feeds the later even flames? Why is it lying there waiting to feed a flare?
Would you say that the heat that is released when the fire ignites, reaches out to acquire more oxygen than subsequent flames have available? Or is something else going on?
It's curious that the change of state from non-fire to fire is explosive rather than gradual -- and yet it's gradual just before it actually combusts.