Anything can happen in just a second while you working. This 1-minute VDO shows you what a catastrophe occurs in just a second.
From the vdo, it seems like the break did not work properly so everything hanging in the travelling block was falling down and hit the drillstring on the rig floor. It did not take long just about 12 second for this case to happen.
This is another event when the TDS and travelling blow falling down to the rig floor.