My guess is he slipped in the shower and hit the back of his head.
A couple of months ago we had a friend that woke up from a nap prior to dinner. We noticed his speech was slurred and his personality had changed--for the worse. We finally talked him into going to the ER where they did various imaging and determined he had hit the back of his head and had symptoms of a concussion. He swears he has no memory of falling, and he would have gone about his evening as if nothing had happened had not my retired nurse wife insisted he go to the ER (she must be obeyed).