What causes transition from smoldering combustion to flame combustion?
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.