Fried Controller

Not seen anything like that, but my guess would be, if it wasn't reversed, maybe it was stalled. Perhaps accidentally switched on whilst parked up against a wall or some such. Sounds like the circuit could have done with some protection.
Of course the other possibility, being on a golf course would be lightning strike.
Big bolt comes down, misses the one iron and hits the controller (like the old joke says, even big G can't hit a one iron!)
Del

I vote for an indirect lightning strike on this one.

Just a wild guess but I can only think of one scenario that would toast all transistors simultaneously. The DC motor generated a sufficiently large back EMF that toasted all of the MOSFETs in one shot. A downhill rolling overloaded cart trying to stop with dynamic braking might generate the correct conditions for this scenario.

A more likely scenario would be a very fast progressive failure and not a simultaneous execution of silicon. A MOSFET or two stopped conducting for any one of a number of reasons. (Most of the failure reasons will be heat related so the transistors in the middle of the heat sink would be suspect.) The cart was still usable but the remaining transistors were now slightly exceeding their preferred current handling. This higher current in the remaining transistors raised their heat production. With one transistor after another removing themselves from the current share there eventually was a load condition that exceeded all of the remaining transistors at once. Pop goes the MOSFET. Finally a technician sees this aftermath and all of the transistors resemble charcoal.

Interesting thought, but I generally find FETs failing short circuit.
Although maybe that's just Murphy's law as a short circuit is the worst condition in the chemical dosing field.
60L of starch dumped into a laundry machine isn't funny... well ok, it is funny, just rather embarasing.
Del

This could be the from the board making contact with the motor due to hitting bump while heavily loaded....This has been reported on some models....The fix was silicone globbed on top of terminals...or it could be from age, just wearing out as somebody mentioned...or it could be due to overheating from driving slow for long periods of time, or uphill, or towing, or unknown variable...

http://www.buggiesgonewild.com/

http://server1.buggiesunlimited.com/phpBB2/index.php

http://en.wikipedia.org/wiki/Waste_Electrical_and_Electronic_Equipment_Directive

Its a rather common issue with high current circuits that have multiple devices sharing the load.

One or more loose connection either externally due to a bad solder connection or corroded terminal or one simply just fails internally to a open circuit or short circuit.

If enough devices go open circuit for any number of reasons the remaining ones get a current overload and blow up in a firey death. Or if one goes dead short it tries to take so much current it literally melts down its internal connections creating a often times violent internal arc which when coupled with having a large inductive load such as a motor in series with it easily creates uncontrollable high voltage spikes and surges which in short order wipe out any remaining devices in a similar fashion until either the system has burnt itself to a crisp or a fuse or breaker blows.

When I have to design a power handling circuit for high current applications I always try to use one large switching device, or at most two devices built into a single component unit, opposed to many smaller and cheaper devices.

Thanks Hitekredneck for the update. It's nice that guys keep others informed as to the outcome of a problem.