fishbird, thank you for your clarifications. To answer the question of how a fire would start, O2 is highly combustible and any spark (from ripping a bumper off a car, for example) could have easily set off a fire in the oxygen-rich environment around the tank.
I'm honestly surprised to hear the man is still alive. Having knocked the tank over with his own body, I'd expect more extensive injuries... not to belittle the injuries he did sustain. It sounds like the majority of them were from the secondary fire, though, rather than the initial explosion.