Flammable gas emitted by sausage
Cured meat should not be emitting methane- or any other flammable gas. That would be an indicator of advanced spoilage and you would notice the smell.

A more likely culprit, in my opinion, would be fat.

Fat burns well, of course, and sausage is full of it. Tiny splatters of fat can become aerosolized and travel a good distance where they would ignite on the burner.