What does the bearing look like? Any damage? There's witness marks ( bearing face) on the retainer pictured and the holes are elongated.
It seems disturbing to me.
EDIT:
since looking at the transmission diagram I could think of something sinister that could cause this, like a damaged/broken tooth on the middle drive gears, this would cause the the middle gear (spiral bevel gear)on the trans shaft to push away
which could force enough pressure to easily break the retaining bolts.
Hopefully it is nothing like that but if it does make a noise after you replace the bolts etc, that's the first place I would be looking