On Friday, July 8, 2016 at 4:31:04 AM UTC+2, wrote:
Remember that "tensile strength" measures a bolt under a longitudinal load. What you are seeing in this instance is a failure under "shear load." Two entirely different situations, and one of the main reasons that aircraft bolts (AN) are not the same as "Grade 8" or the European (DIN) equivalent. High tensile strength bolts often exhibit less than desirable brittleness under shear load.

I think Mark is close to the root cause of the failing bolts. It is not the strength of the bolts itself that makes in brake (sounds funny, I know).
In the bolt-configuration in the initial post is the function of the bolts to hold the Al-Ko tongue tight to the square bar, the friction between the tongue and the bar makes the strength.
If the friction coefficient is low (fat, grease, dirt) or bolts is not tight (loosen by vibrations, deformations etc) is the friction between the tongue/bar low and a shear stress occurs on the bolts. The bolts are not dimensioned for this and will brake by fatigue.
This is a common problem on Cobra trailers(the nose-cone / spare wheel holder), the bolts a not tighten enough from the factory and / or they vibrate loose on the road.