Where it comes from is the charge. Its simple physics. Remember the law of physics that any action results in an equal and opposite reaction? You load a lead sled up with weight and set off a stiff charge - something has to give. In this case it was the scope. I've also seen in one of the other forums where a guy was shooting his 416 with 100 lb of weight (like on the TV ads) and cracked his stock.
When shooting off your shoulder, your body acts somewhat like a shock absorber - restricting and cushioning, but allowing movement.