I believe it's a transport that lost a wheel. That is the tire from a typical flat bed transport, possibly a logging truck.
The odds that it rolled off and actually hit that guy are still quite incredible. I would imagine if a tire that big falls off a truck it would do so slowly and it would drag for a bit then bounce around, and not actually roll perfectly, but it did.
I'm guessing the put the pump back on to make sure it stops pumping in case the hose got damaged, then went for help. It's bad enough to get hit by a tire like that, you don't need an explosion at the end.