Can someone explain the science behind this? Usually we think of explosions occuring when something combustible is under intense pressure, like when it's in a small, confined place (like a firecracker, gun barrel, etc.). Why, out in the open, did it explode, instead of just go up in flames very quickly?

I know Kerosene and Diesel wouldn't have done that. Gasoline burns faster. But again, why not just a faster burn/faster lightup, instead of an actual explosion?