Apparently, he was not wearing a safety harness while in his chariot.
From The Independent:
_The mystery behind the sudden death of Tutankhamun, the boy king who ruled Egypt more than 3,000 years ago, may have been finally solved by scientists who believe that he fell from a fast-moving chariot while out hunting in the desert. . . .
“He was not murdered as many people thought. He had an accident when he was hunting in the desert. Falling from a chariot made this fracture in his left leg and this really is in my opinion how he died,” said Zahi Hawass, general secretary of Egypt’s Supreme Council of Antiquities._