No one really knows for sure. There are a few theories floating around still. Other people just say, he did it because he felt like it. Read on if you are interested:
Paul is dead" clues
Main article: "Paul is Dead" clues from Abbey Road
The cover also supposedly contains clues adding to the "Paul is dead" phenomenon: Paul is barefoot, with eyes closed, out of step with the others, and holds a cigarette in his right hand, though he is left handed, and the car number plate "LMW 281F" supposedly referred to the fact that McCartney would be 28 years old if he was still alive. "LMW" is said to stand for "Linda McCartney Weeps." The four Beatles on the album cover, according to the "Paul is Dead" myth, represent the priest (John, dressed in white), the undertaker (or perhaps mourner) (Ringo in a black suit), the corpse (Paul, in a suit but barefoot—like a body in a casket), and the gravedigger (George, in jeans and a denim work shirt). The man standing on the pavement in the background is Paul Cole, an American tourist who was unaware that he was being photographed until he saw the album cover months later.