There is no exact reason the bearing failure occurs, but there are many things that have shown to be damaged.
Lack of lubrication from a sealed bearing
The bearing installed is factory sealed at both ends with grease, which will not allow any oil to penetrate the balls inside the bearing. The factory installed grease when overheated or after time will liquefy and melt out of the bearing seals and when this happens the oil is blocked off from lubricating the bearing and eventually will come apart.
Small threaded centering bolt
The centering bolt installed by the factory has been machined down to a smaller size for accepting the nut. The end of the centering bolt we have seen bend and eventually snap the end of. The factory installed bolt has also been machined down to accept an inner seal which can also be a weak point. The factory bolt is not heat treated.
The centering bolt nut
The factory installs a standard small flanged nut which holds the centering bolt, bearing to the centering flange. With this many parts counting on a nut to hold it all together a quality nut should be installed.
The factory has tried different version themselves
All three have been seen to fail
This Porsche Intermediate shaft FAILURE!
The motor will need to come completely apart now, there is no "quick-fix" for this motor.