He tried to stop the cover from falling onto another crewmember by blocking it. The cover was too heavy for him to control and it fell on him, fracturing his arm. Little consideration had been given to the possible hazard created by this unsafe act.
Safety is everyone's responsibility.
When you see an unsafe act or condition, stop and correct it. That is the smart thing to do. Do not take short cuts and sacrifice safety at any time. More time is lost when an accident happens than when we take a few moments needed to correct a hazard when it is first noticed. If any items cannot be properly secured during rig-up or other operations, ensure that they are kept in a position that keeps them from falling or creating other hazards.