One can imagine the chain of events that leads to a situation like this. A truck, airborne, between two animals the size of buildings, neither of them even acknowledging that the truck exists. It is the sort of image that sounds impossible until you are inside it, at which point "impossible" becomes a very lazy word.
The first worm had thrown him. Its body had been rising when his front wheels caught the surface, and he had ridden that for about four seconds before the worm dipped and let him go into a brief, unplanned flight.
The second worm was moving beside the first, its back thirty meters ahead and slightly below his current path. He had maybe one second before the question of whether he landed on it or beside it answered itself without consulting him.
He landed on it.
