Well, first is the fact the guy did it to himself. Second the guys in the video are cracking up and third, have you ever laugh at a clown at a circus? Or maybe at a joke at someone else’s expense? Humans laugh at things which are so painful in some certain circumstances to relieve the mental pressure of a sad event which shocks.
OK, got it. I thought it as a trick they played on him. I still find it hard to laugh but I can see why others would.