It was definitely a car crash. He had an accident in his Aston Martin. Which is why the Paul-Is-Dead-Really-He-Is people always refer to that little car propped up against the doll wearing the Stones t-shirt on the Sgt Pepper album sleeve. The scar on his upper lip wasn't Paul's, apparently, but the impostor's, who'd had plastic surgery to make him look just a little bit more like Paul. Hence the moustache. And the other three grew them too, so it wouldn't look suspicious.
I just LOVE a good conspiracy theory, crap or not! [:D]