Inb4: I don't work in the industry, my knowledge is limited to a faint memory of a college course on fluid dynamics.

If I recall correctly, high pressure ignitable stuff can spontaneously turn !!FUN!! in absence of heat if it is suddenly relieved through a pinhole. Basically jet is followed by a ring-like zone where the stuff mixes with oxygen. Jet creates tiny zones of very high temp, thus igniting the mixture ring that follows.