From Muppet Wiki

Hurling Day, the titular occasion from Episode 103 of Dinosaurs, is an ancient dinosaur tradition. Established by a wise but aging dinosaur chief during the days when dinosaurs roamed in tribes, Hurling Day served to prevent elderly tribe members from slowing down the pack, and allowed them to depart the mortal coil with dignity. Thus, upon turning 72, old dinosaurs were to be ceremonially pushed off a cliff by their closest family in a loving and respectful manner. Hurling Day soon became popular, however, as a means by which husbands could take revenge on their mothers-in-law.