Given that the manufacture of stretch limousines, aftermarket as they are, might possibly not be as subject to regulation upon the use of flammable materials and often do involve fairly elaborate electrical components such as sound systems and interior light displays, I suppose that an electrical malfunction could have set the whole thing off, but rapidly enough to cause fatalities to a majority of passengers? That was more akin to an interior fireball, imho, so I still lean to a flammable substance, an accelerant of some kind.
That and some flame retardants can be simply washed away with plain water. I’d guess a limo occasionally needs to be hosed out. Do that a few times, have an ill timed Everclear spill...