General Disarray is exactly right. The dynamic loads are what kills them, not the static loads. In other words, 50lbs on the bumper in the driveway is 50lbs. Whoopdeeedo. 50lbs on the bumper on the highway hitting bumps is magnified by the acceleration. So that 50lb static load could very easily be like a 300lb dynamic load on big bumps. (could be more or less) Not only that , it gets repeated over and over and over. Aluminum doesn't like fatigue loading either. So you hammer the thing apart.
I do believe that the OEM rear connection could have been better. But this is the way they are.
And for a motorcycle, the real problem is the handling. I was looking to build a new frame for my own trailer that would absolutely not bend. However, the problem of handling still comes up. You want the mass to be centered just ahead of the axles. You put 350lbs on the back bumper and its 12' aft of the axle centerline. Every time you make a side to side oscillation, that weight out on the end is trying to wreck you.
It's like holding 25lbs on the end of a barbell vs. holding the 25lb plate in your hands and trying to dance the twist. WAY easier with the plate in your hands.
In EngiNerd terminology, we call that "polar moment of inertia." You want the mass centralized.
You could always build a new frame, enlarge the door to about 40" wide, and park the bike in the trailer when you travel
I actually saw one that did that....it was pretty cool!