What would cause the rear wheel on a kawasaki vn800 to lock up?

It could be that one of the Pistons in your engine have locked. This can happen if the motor chassis breaks, or the motor gets too hot and the piston expands due to the heat, gets stuck and damages the engines internals (and if unlucky the valves as well.

I had this happen to me once on the Highway. All of a sudden, the back wheel of my Aprilia blocked while I was going 70 Mph. Not a good experience. In my case it turned out the cooler had failed, and the engine overheated.