the big explosion was the satellite hitting the ground and it's ~2 tonnes of hypergolic fuel exploding.
the initial flash-fire looks like it's from outside the second stage where the LOX umbilical is connected.
one can only speculate that either there was a failure in the umbilical/coupling or some contaminate came into contact with the O2 venting, a lot of things get very incendiary in a pure O2 environment.