That's not necessarily so. He could have had mechanical failure, or it could have been driver inattention—talking on a cell phone, daydreaming, falling asleep. You don't know.
To parallel it in court, when I was doing impaired drivers on a very regular basis and mentioned watery bloodshot eyes, the first thing that came out of the defence was a question about how smoke could cause watery bloodshot eyes, or how being tired could cause watery bloodshot eyes, or how allergies could cause watery bloodshot eyes. The answer is that certainly a whole lot of things can cause watery bloodshot eyes; by itself it's just a clue, but if you tie it with everything else, then it becomes a building block that allows us to go forward with a charge.