Just related your experience to my son. He has had this unit for several months for his motorhome. Love's it. No real problems, but he says that if you are grilling burgers, at full heat, with the drip pan opened for direct flame, you can expect flame ups. If the lid is closed you may see flame coming out of the drip spout. He has not had any flame backs into the hopper. He does think that direct flame for burgers and steaks could be done with lid open, but only if you plan to just use the area of open flame. He has not tried this tho.
He agreed that with the lid closed for shut down, that there might be back pressure that could account for auger and hopper fire. \\
I have another thought. Since the hopper is small and the auger short, is it possible that there were few pellets left in the hopper and thus when you shut down, that there was less obstruction to back flo into the auger/hopper? The fan is on full during shut down, and with the lid closed, fire might backup easily? Perhaps if hopper had been fuller, it would have reduced backflow.