OK, here's about the best I could come up with:
I did a test burn today and did find traces of pellet dust on the chute, therefore, on a very long cook as I did on the problem day, I might have had a lot of dust build up on the chute. I have no way of knowing since is was just going by meat internal temp. and never opened the door.
Also, with my Traegers, the fire pot stoker air is like a jet engine and any spend fuel is blown into the chamber and doesn't stay in the fire pot. The FEC on the other hand has a gentle breeze stoking the fire and after 14 hours would more than likely have a firepot filled to the top with spent and burning pellets.
This would bring the flame in contact with the chute therefore igniting the pellet dust which leads to the auger.
Even though the pellets I was using, BBQers Delight, did not look dusty, they did leaves a trail. I did observe that many were longer than Traeger and they may have been crunched up in the auger.
My practice from now on is when I need to burn longer than 6 hours, I'm going to open up and wisk the shoot down.
Anyway, that's my story and I'm sticking to it.