If you've unluckily experienced a pellet grill/smoker fire. There are two possible scenarios.

It could have been a pellet hopper fire, which is quite rare but can happen. This is where the fire travels back up the auger tube and into the hopper. If you keep the hopper lid closed so there is little to no air movement up through the auger into the hopper this should prevent such a scenario.

The most common pellet grill/smoker fire is due to a grease fire. Pellet grills/smokers have a large grease tray/drip pan over the burn pot that needs to be kept clean, otherwise, a grease fire is going to happen sooner or later. Before each cook, make sure the grease trap is clear so the grease can exit into the grease collector/bucket.

If you have a pellet grill with direct flame access/flame broiler you need to be particularly careful to avoid a grease fire by keeping the grease pan/tray clean. For instance, if you have previously smoked a large fatty piece of meat (brisket), you'll want to make sure the grease tray is clean before using the direct-flame feature for searing burgers/steaks etc.