What causes transition from smoldering combustion to flame combustion? What causes the transition from smoldering combustion to flame combustion?

From what I have read about it is that, smoldering combustion gives off a lot of flammable gases, primarily, Carbon Monoxide, however, the lower flammability of carbon monoxide is 12.5% and it takes about 2-3 hours to reach a concentration of 1%, it is lethal to humans in concentration of 1%.

There have been reported experiment studies, where smoldering combustion transitioned into flame combustion, as quickly as 22 minutes. So, there must be some other cause. Carbon Monoxide is, I guess not the reason.
Burning in open air isn't a steady state process, but a very dynamic one. Likely when the fuel is smouldering there isn't enough of a convection current in the air to bring in fresh oxygen. As the smouldering continues something about the geometry of the area smouldering changes and fresh oxygen is brought in by a strong convection current. At that point the fuel bursts into open flame.