The most common cause is binding brakes/ wheel bearings (or hand brake partly on, although our Eriba the handbrake is either on or off, no in-between), or electrical - especially the fridge. But a really sad loss, at least he was able to keep the family safe and decouple the car.
Dave
PS At every stop I take the opportunity to test the temperature of wheels and tyres, and now I have wireless temperature sensors on both wheels