Ah I thought @Dei was saying it had come off the semi that just went by, which I can tell it didn't.
You know, up until now I always thought the dual wheels on trailers and such were two conjoined rims, I didn't know they could be removed (or go wandering) separately.
As for why it got launched, my guess is that the vehicle rolling over it compressed the tire, squashing it and storing energy the same way a basketball does when it hits the ground at the nadir of its bounce. As the vehicle rolls over the tire, the weight is no longer enough to keep it compressed, so it rebounds. Further, you can see that the road tire gets up under the driver's-side rear tire, which probably added its own compress-spring mechanic, not to mention any additional forward spin it might have contributed.