the owner is in my golf group. no one was hurt and the cart was fully covered and replaced..

Ooh...
A busted tire pales in comparison !

IMO Sound Like direct short in one of the batteries or wiring. Everybody knows that batteries product hazardous gas than can explode if enough contained and spark occurs. Rare due to designs are well ventilated.

Probable cause may have been corrosion main power cables or wires to elect motor melted wiring started fire. Due to the heat generated by the melting wiring plastic started burning throughout the interior of the cart. IMO that's why it's very important to look for corrosion and be able to tell if wiring being eaten up inside by the corrosion. When this happens it reduces the Amp that the wiring or cable able to carry, result is heat, enough heat and puff FIRE!

I don't think the fire investigators have concluded what the cause was, so unless someone calls them to see if they have reached a conclusion.......don't jump to conclusions please.

Better on the cart path than in their garage!