I'm thinking it was caused by one of his nocturnal "business associates" letting him know that they ain't pleased to find out all those promises he made as Senate Majority Leader, well, now he can't live up to them.
Interesting coincidence, Harry also had a similar accident in May, 2011, where he suffered a black eye and a dislocated shoulder. He described the injury as having occurred when he was "out running".
I never realized that decrepit old fool was so interested in physical fitness. Or not.