Looking at the picture of the bolt that is a shear break, not a pull break. That means that forces are acting on the bolt through the threads, not with the threads. The deck is hitting on something not being pulled down.
Or a fatigue fracture due to vibrations while the deck is running.
Vibrations do not need the deck to be hitting something.
If the hanger bolt is in tension from the weight of the deck then the hanger is being stressed in 2 planes which drastically reduces the fatigue life of the hanger.
Any further diagnosis of the hanger will require some high resolution photos of the fracture surface.
As it has also fractured through the thread root, the thread may have been badly cut with too sharp a root which becomes a crack initation point.
The bolt also shows fretting signaling that it has been hitting something fairly regularly.
If the D bolts on JD decks are put in backwards a similar fracture can happen.
A D bolt puts a bending moment on the shaft by virtue of the fact all of the weight is on one side of the bolt.
If the bolt is subject to flucuating loads along its length for example slightly more load as the blade swings past then a decreased load till the next side passes you will be getting a fatigue cycle 2 x blade speed.
So the whole thing could be nothing more than an out of balance blade, either in weight or airflow or both or a slightly bent blade or using a different blade from the one it was designed for , or running the blades too fast.
There are a myriad of factors that have to be considered in a proper failure analysis.