What causes this?

[DrZaiusDrZaius]

Timberline 850, got a high heat warning and raced home only to find this -- more pellets had piled up and ignited under the heat baffle. Had it running steady at 275 when I left the house.

 

[BobbyQ]

Could be a controller problem or bad thermocouple. Running in open loop which means the controller is not getting correct feedback on the pit temperatures so the auger keeps feeding pellets. I would contact CS and perhaps send them the photo.