Since you say you have little or no horizontal pipe for fly ash to settle in, let me ask this...do you have a barometric damper?
If yes, was it wide open a lot yesterday when it was very windy outside? ...or was it mostly closed?
My thinking is that your baro is too small or is just set for too high of draft and the stove was experiencing a high volume of draft and drawing excess air from the hopper.
Had you not caught it it would likely of eventually burning back into the hopper.
You done good checking CO and getting it shut down right away.
Manometer will tell the facts.