Answer:
The idea was suggested in The Lives of John Lennon by Albert Goldman, and was repeated by one of Sutcliffe's siblings later, that a drunk or crazed Lennon was the source of the head injury believed to lead to Sutcliffe's death. (History holds that Sutcliffe was attacked by thugs after a Beatles show in Liverpool, and that Lennon had fought them off.) There appears to be no solid evidence or witnesses, and later examinations of Sutcliffe's medical records determined his death could have actually been caused by an inherited condition.