Depends upon the ‘fire’ you are referring to, but let’s say you are referring to a common match.
What you see initially is an accelerant being ignited and burned quickly. When the fuel is gone, the cardboard of the match continues to burn, but at an even rate of combustion, since there is no accelerant.
To watch how most fire burns, minus the accelerant, take a common cotton rag, and then use a magnifying glass to get it to burn. Make sure it’s clean first. You’ll find it smolders, and then you have to blow on it in order for it to turn from an ember to a flame.
That makes even better sense — the flare is when the accelerant gets eaten up, which happens rapidly? and then the subsequent flames are the fire eating up whatever else is there?
Again, basically, fire looks lively but is actually moribund? Or non-sustainable as people are lamentably inclined to say these days.