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Stick burners are all about having good flow through them to cook good meat. I try to cut my wood to size when using one so that it has a free burning fire that keeps the temp in the range I want it. Might be hard to picture this but to much wood makes to big of fire that tries to get way to hot...when you close down dampers to gate fire back it smolders and produces thick smoke making bad tasting food. To small of a fire leaves intakes wide open with not enough heat. Ideally if you can get the smoker to temperature and hold it in the range you want with dampers open letting fire free burn it makes the cleanest smoke and best tasting food. It's trial and error though until you find what your cooker likes and what range it likes to run. In my 500 gallon butane tank smoker the firebox is kinda undersized to the smoker body but I use it to my advantage. Once I get the draft going, I can open the firebox door fully and let it free burn adding a split every 30 min or so and it stays between 230-270 so it is a ton of intake air flow but again it is within my desired temp range and has a clean free burning fire. Each smoker is different but the cleanest fire comes from one that isn't deprived of fresh air.
Lots of possibilities. Is it a reverse flow? Accumulated grease build up hot enough to ignite, under seasoned wood until it gets hot enough, creosote buildup catching fire, anything foreign made its way into firebox (oil from seasoning possibly getting into firebox), giant wasp nest in smoke stack. Last one sounds pretty random but it actually happened to me once. I knew it was in there though because they came after me when I went to open the door...so I snuck around to the firebox after they settled down and burned em out.