Overfire in an NC30

Today I had loaded on top of an existing bed of coals-a short 6" round of walnut + maybe 6-8 - 2" rounds of walnut, willow & maybe some misc stuff. The wood was normal dryness. The fire seemed to be well established like on any other day so I left it. I closed the air control to 20% at the most. (on an NC30 this is where the brass knob is just even with the ash lip). This is where I almost always let it cruise at. 30-45 minutes later I went downstairs to check on it and started to smell that hot overfire smell. The flames where whirling in there. My flue temps (2-1/2' up the 6" stack) were at 900 and the thermometer mounted on the side was at 650. I shut the air off, turned the blower to high, took a look at the chimney outside and within 5-10 minutes it was back to normal. I don't think I have ever had it this hot. What would cause this? I could understand if I forgot and left the air control fully open, but when it happens under these circumstances I find it a little concerning. I could have easily left the house or gone to bed with the same situation.

Whatever the reason I am glad there is some reason? Not knowing would have driven me crazy. I must have gotten the right mixture of dryness in this kindling to set it off. I have done this many times before with no problems. I seem to have an abundance of 2" pieces from some tree clearing that I have done. More than I could use for kindling alone. I will definately be more careful in the future when I use this. I mistakenly thought that setting the air control in this nearly closed position would prevent a runaway fire.

I was probably more concerned with the 900+ going up the stack. Yes, I have the temp on the side as high as it will go and in the center. I don't like it on the top because with the blower air going through it I felt that it was giving me inaccurate (too low) readings.