Air flow is the issue. If there is enough air, the grease will flame up. I see this frequently on my kettle when I'm not using a drip pan. As soon as I open the lid, the flare ups start. As soon as the lid is back on, the flames go out. If you have flare ups while the UDS is closed up, you have too many openings that allow air in. If you have flare ups while the lid is open, close the lid!