Possible the tire was already hot, and when it stopped additional heat from the drum or wheel soaked in and pushed it over the ignition point. I dont do it every time, but I have the habit of walking around most every time I stop, touching the tread and visually looking at all the tires, and touching the hubs feeling for heat. More than once Ive caught a failure in the making, and was able to fix it before it turned into something ugly. A nail in a tire, caught before it goes flat, can save a lot of grief.
This is a prime example of the need for a dash mounted tire monitor system. It would have warned the tire was overheated long before it got hot enough to burn, not only preventing the fire, but saving the Bus itself. Of course it would also prevented Bob from getting a sweet deal.