Back burns are when the pellets inside the auger tube and potentially in the smoker box are combusting. This is evident when smoke starts to come out of the actual pellet box. To avoid this common occurrence, buy a smoker with well built temperature control and fan systems. If the day is windy, angle the smoker so the wind is not blowing into the direction of the auger and pellet box. And keep the fire box clean. Most commonly this occurs when ash is building up in the fire box from prior cooking and the pellets are bunching up near the auger.