Pannier bags/luggage on fire - Malmsbury N7 yesterday

Does anyone know what happened on the N7 just after Malmsbury (Cape Town side) yesterday at about 15h00? I was driving back home and passed two bikers (one on an older Africa Twin) who seemed to have shed a series of burning luggage items along the N7. First I thought there was a protest because there was burning items in the middle of the road for about a km until I passed the bikers. The last guy had parked his bike with a burning item by his side stand and was walking back to his mate who was a few hundred yards back. I tried to gesture to him the risk to his bike but I think he had other thinks on his mind?

It was pretty crazy the fire a burning sleeping bag makes!! What could have happened?? Exhaust touching luggage? Leaking fuel/cell. Luggage touching the rear wheel? Has anyone seen such a thing?