Several readers have offered explanations along these lines. Sounds plausible:

Jonah,
The one thing that will cause any and every skydiver to fly into a violent rage is the implication that there is anything remotely unsafe about their “sport.” Putzboy isn’t mad at you because you were flippant about skydivers dying. He’s enraged that anyone would ever suggest that skydiving has any risk associated with it. He’s safety conscious – you’re uneducated – blah blah blah.
When I visited my sky-diving friend and his brand new titanium femur in the hospital, one of his comrades-in-unnecessary-risk accosted my wife for suggesting that this sort of thing can happen. Since he had been walking with a cane much of the year prior to his latest mishap, we thought it was reasonable to state what had already happened can happen. His fellow jumpers thought profanity was a reasonable response.
So watch yourself – you have awoken their wrath.