The lid popped (lid burp)

A loud pop and the lid jumped

Traeger support, "Lid Burps": a lid burp is unburnt pellets and gas in an over-full firepot igniting at once, usually after a bad shut-down or a flame-out.

• Step back and wait 2 to 3 minutes before you open the lid.
• Then, cold: clean the firepot, empty the hopper, vacuum the auger tube, and use fresh pellets.