The .50's that I have fired had some issues with opening the bolt and extracting the spent case. I would just give the bolt handle a really good yank and it usually came free. I could see tapping a bolt handle back with a dead blow.
IIRC back in the early days of BMG target rifles the shooters would hit the gun with a hammer if there was a hang fire. This was to ensure that the gun was actually in a safe condition to open the bolt. I imagine a BMG going off in half battery would end up close to what happened to this guy.