It looks to me like the welds broke, rusty? Thats a shear break, I cant really tell, but in the pics it doesn't look like good penetration in the weld. Thats what happens when you do production welding, some of the welds get slapped on and not properly welded. Glad to see your prop was not damaged. When I was 22 or so, I for got to clamp the jet ski trailer hitch to the tow ball, I hit my first bump and thank GOD I had the chains correctly attached because it came off and slammed into the back of my truck. No damage to truck or trailer, just my pride!!!