Unless the people were storing explosives in their house it was a gas leak, can't think of anything else that can tear an entire house apart like that. Stored gasoline or a boiler explosion would just take out one room.
Wonder if they had a new dryer or stove installed? The last few of these I've heard about have been right after work like that was done. I guess these days you've got to do your own leak checking after you have a new appliance installed, brush soapy water on all the fittings and look for bubbles. And do it for a few days, a fitting that wasn't tightened all the way might loosen up if it's on something that vibrates like a dryer.
I've thought about changing out my electric stove for a gas one, my folks house had a gas stove and it is nicer to cook with, but I've gotten so used to using the glass cook top as extra counter space I don't want to give that up. And the glass cook top has pretty good temperature control, not quite as fine as gas but much better than a bare coil type range.