Firstly your shoulder would follow the rotation for as long allowed. Then something would be dislocating, but whether it would be the head of radius or your shoulder I cannot tell. From there you get a whole lot of soft tissue injuries.

If you'd somehow continue to pronate (rotate your hand "inwards") and fixated your upper arm, you'd probably get a fracture of the proximal ulna or distal humerus, as they are rotationally fixed to eachother.

Note, I have not tested this. This is only my intuition, as someone who knows a bit of anatomy and medicine.

My first thought is it would be something like this (https://radiopaedia.org/articles/chauffeur-fracture?lang=gb) . According to radiopedia, that fracture came from the crank handle, of an old fashioned crank start car, reversing and impacting the dorsum/ back of the wrist. Additionally they use terminology, e.g. dorsiflexion? instead of extension, and abduction? instead of radial deviation which is a bit different than I was taught, and taught to others. Tho' that mechanism doesn't seem to be what I recall from school. In the off chance that I'm correct, as I recall the fracture occurred when one wrapped one's thumb around the crank handle, such that when the engine started the thumb would provide leverage against the radial styloid process. I've met people who own such antique cars, and have never heard of any of them making this mistake. So to answer your question, it's not random, few things actually are. It's the radial styloid.