DIAGNOSIS That's a minor flashback. A small amount of gas accumulated in the cook box before ignition. It's common if you wait a few seconds between turning the knob and the igniter sparking.
FIX Follow the proper lighting procedure exactly. With the lid OPEN, turn the control knob to IGNITE/HI immediately after you start pushing the igniter button. Do not push the button first, then turn the knob. The action should be nearly simultaneous. This minimizes gas buildup.
IF THAT DIDN'T WORK Check for obstructions in the burner tubes (spider webs again). A blocked burner can cause delayed ignition and gas buildup.
HEADS UP Always light with the lid open. If you ever get a significant fireball or the whoosh is violent, turn gas off immediately. You likely have a serious burner obstruction or leak. Do not use.