Any constrictor over 8 feet in length should not be handled without another knowledgeable person present, to prevent accidents. This is part of safe handling practices.

I understand the boa there is actually 9 feet.

Obviously you don't put a constrictor around your neck. It didn't attack him...it tightened up, and cut off his airway, then the snake probably freaked out due to all the activity and just tightened up even further.