Over pressurized gas lines.
The main lines are under a LOT of pressure. As it feeds out there are regulators to bring the pressure down to what the smaller lines can handle, usually two, one main and one backup. What experts are saying is that both failed somewhere and the regulators at the houses couldn’t handle the increased pressure.
Some places blew up, etc, some didn’t. It takes a special mix of NG and air and spark to create a blast.
Somewhere in the range of 10,000-15,000 ppm :-).
I squeaked by on a couple of UST removals when the LEL (lower explosive limit) meter went south but I was able to demonstrate well below 10% of LEL with an organic vapor meter.