Looks like Darwin's law of selection to me.

Pretty sure that the slightest wiggle of the trailer you'd be able to feel in the car...

Pretty sure that if you felt the slightest wiggle of the trailer you'd look in the rear view mirror...

Pretty sure that if you looked in the rear view mirror and saw your trailer moving oddly in the slightest, you'd at minimum let off the throttle, and more likely use the brakes to slow down.

Either would have prevented this.

Common sense fail... hello, Darwin. Didn't expect to meet you here.