Tragic is an understatement here, so sad.
I'd say there's no excuse for it, but, I suspect some of us have forgotten something simple before and just got lucky no one got hurt as a result.
I'll fess up and admit one time I forgot to put the pin in my tilt snowmobile trailer. Half way up a steep grade the trailer flipped up and started swerving side to side behind my Jeep. I am fortunate that no one was coming at us the other way!
As for the hitch pin, I have used a Master Lock pin for 15+ years, through all weather conditions, including salty New England roads in winter (we snowmobile too). These are stainless steel and have rubber seals. It fits tight to the receiver so that you couldn't possibly cut them off either.
I picked this one up at West Marine after I got my GMC, as it was the only place that had the wider pins for my Class V hitch.
EDIT: Here's a link