Just as a matter of curiosity why would two big holes be burned in the top of the fuselage ? Seems that’s where carry-on luggage would be secured in the overheads.
Obviously there was a fire started during the landing with engines ripped off and fuel lines exposed and leaking. It would seem that the interior fire started out small since the passengers were able to evacuate but probably spread along the top in the contained cabin - just like a house fire will burn ceilings first before expanding to the floor.
It seems more effect than cause of any type.
As posted above this approach was troubled well before impact. Though the passengers were unaware of issues until at or near impact this whole approach seems to have been bad. It does not appear that any sudden or catastrophic event (i.e. explosive or on onboard fire) was the cause.
SFO emergency crews seemed to have done well in dousing the flames quickly also. Hats off to them.