The car had been partially backed out of the garage. I think the door was opened and the car partially backed out.
Two things have to happen to have the car actually backing out and have it stall ~ (1) The driver has to stop pressing the accelerator and (2) the engine has to stall.
With automatic transmissions being so common these days it is easy to forget that not too long ago if you suddenly died at the wheel, you'd rapidly slow down and your engine would shudder and die with you. Now, they just keep on running. This car stopped cold.
So, let's say the engine stalled ~ sudden consumption of all the oxygen in the vicinity by a big fire in the immediate vicinity would do that. That means the fire, as big as it got, roared to life instantly as she was backing the van (if she was, in fact, backing the van). That's remarkably like an explosion.
Final reports will tell us whether the vehicle left tracks ~