I think part of the problem is that the snap up brackets are not square to the frame.
So when when you tighten the pinch bolt, the snap-up get pulled flush to the side of the frame, but then it's only touching the top of the frame at the outer edge.
My guess is, since the snap up is only touching the top of the frame at the outside, that the pinch bolt is taking on a lot of the force from the spring bar that is really supposed to be handled by the bracket itself.
If enough load is present, the the pinch bolt will start to bend the frame (or the bracket will bend - or both) and the snap-up will begin to tilt down even more as the frame gives way. Once that starts to happen the snap up starts applying a point load on the side of the frame at the bottom of the snap up. So the snap up only contacts the frame in 3 places once it starts to go 1) the pinch bolt, 2) the top outer edge, and 3) the center of the frame on bottom of the snap up.
#1 and 3 are basically point loads - the bottom of of the snap up has 2 points that contact the frame and the pinch bolt obviously has a small surface area.
If the snap up goes over far enough the keeper pin for the snap up takes all that load as well and begins to bend.
Is this speculation even in the ballpark?