I happen to work in Orthopedics.
What poundage were you pulling?
A stress fracture is a possibility, but a complete fracture is definitely unusual.
There are 4 muscles that attatch to the top of the humerus(arm bone), these are the Rotator cuff muscles, which can definitely tear, and take a piece of bone with it. This is an avulsion fracture.
Often, there can be a cyst in the bone that can weaken the bone, and you would not have any symptoms prior to the bone breaking.
Keep us informed as to what they find on the MRI