My theory - based upon absolutely nothing but wild speculation — is that the fire started due to careless smoking by one or more of the passengers.
“Something flammable in the trunk?”
That’s what I’m thinking. It was a Lincoln Town Car type limo. To hold 9 passengers it must have been a stretch version. It must have started with flammables. The driver could have kept gasoline in the trunk that leaked or it could have been something the passengers had with them. Or as others have mentioned freebasing. In any case it was a terrible way to go.