Welcome to TBN, NTJ!
When you get a good look at how the prop shaft is configured, you'll understand why/how this can happen.
There's a flex coupling at the front of the shaft that is splined to the pulley that is bolted the crankshaft.
This is the pulley that drives the fan belt. Here's my story about replacing it:
http://www.tractorbynet.com/forums/kubota-lawn-garden/268623-g5200-hst-prop-shaft-spline.html
At the other end of the prop shaft is a very different kind of flex coupling.
This one uses a thin, flat piece of reinforced plastic - sort of resembling circuit board - to transmit torque, while allowing some misalignment with the HST input shaft. Why they use this design, I can't say. I suspect they use it instead of a shear bolt, to "fuse" the HST driveline, so an over-torque can't get transmitted to the HST, and vice-versa.
I was amazed that the thin plastic disk on mine has completely outlived the spline on the shaft, which wore almost completely through the steel spline teeth. There's a good photo of the plastic disc, just to the right of the HST fan, in this thread, started by fishfarm:
http://www.tractorbynet.com/forums/kubota-lawn-garden/279832-replacing-g5200-transmission-fan.html?highlight=g5200#post3307965
I'll be interested to hear which end of the shaft let go, front or rear. By the way, this is an excellent time to replace the radiator fan belt, as it requires separating the prop-shaft. Also check the state of the splines and the HST fan for any missing or damaged blades. I found that removing the mower deck and the fiberglass top deck was required to adequately see and access everything. But I'm kind of big and clumsy.
-Jim
