We used to have bonfires of 8 foot logs after soaking them in 15 or 20 gallons of gas but we expected an explosion and lit them with roman candles from a considerable distance.
In this case it sounds like it was pallets which means air spaces.
Air spaces which would act to contain and confine the gasoline vapor, instead of the vapor dispersing into a less-harmful rising fireball.
I think the problem here is that it was two stories high, I’m guessing that’s what, 25-30 feet?. They created one huge cavity that just kept who knows how many cubic feet of vapor trapped inside. We built bonfires with soaked wood too, but all we got were some big wooshes. Not once did our bonfire ever blow up and scatter over 100 square yards. That’s a helluva big boom.