A shear pin is different from a regular bolt because it is designed to break, or “shear,” which in this case is a good thing. Shear pins are made with a weak point that snaps if a jam occurs between the auger and the housing.
Let’s say a rock gets between the rotating auger and the housing. It stops the auger, but the auger shaft, which the auger is connected to by a shear bolt, wants to keep turning. If the jam by the rock is too severe and outmatches the force supplied by the gear case to the rotating auger shaft, the shear bolt breaks, allowing the gear case and the shaft to continue moving, undamaged.