Here is what I think must have happened to produce the gas explosion. I don't have enough evidence to suport this, except that no other scenario makes sense. We know that there were both gas and water pipes down there. After the explosion. the crater filled with water from the broken pipe. It's possible the water pipe was leaking before the explosion, creating an underground crater, which would have eventually led to a cave in, if the explosion had not occurred first. We know these things occur frequently. Now lets assume there was a gas leak in the pipe within that crater, causing it to fill with a combustible mix. Now all we need is an ignition source to complete the disaster. This could have come from a car exhaust or even a discarded cigarette, igniting the explosive mixture leaking up to the surface.
You can't explain that explosion by a ruptured pipe. That might produce a big flame, but not a crater-producing explosion.
Some of the news media attributed the water in the crater to runoff from the fire hoses. That seems implausible, since we know there was a water main down there.