Broken disc mower, help please!

Hey everyone, I was mowing yesterday with my new Holland 1410 discbine. I broke one of the caps the holds the u joint in place on my drive shaft. The u joint itself has no damage and shows no sign of wear. Any idea what would cause the cap to bust off? Did I turn to sharp and it pinched it? The mower had been working fine with no problems prior to this. Any ideas? I'm new to haying as a lot of you know so don't assume anything....

Broken disc mower, help please!