I'll put a potential grease fire to one side, as that would pretty obviously be the cause of a pellet grill exceeding its maximum temperature setting.

If your pellet grill is exceeding its maximum temperature setting its most likely due to a faulty RTD probe. That's the temperature probe that lives inside the cooking chamber and communicates with the control panel.

RTD probes are quite sensitive and when they fail or they are very dirty (coated in grease) the controller could keep feeding in pellets try to reach the set temperature not knowing the actual temperature is above the set temperature.

The first thing to do is gently clean the RTD probe with warm water and soap/degreasing agent. If the pellet grill is still overshooting its set temperature, then the next cheapest potential solution before changing the control panel is to swap the RTD probe.