So, I’ve been pulled over a few times in my life. Not many, but a few. And I’ve also been in a couple of cars that got pulled over. And let me tell you, if you were actually doing something wrong, the officer doesn’t make any small talk, just straight into “I clocked you doing 70 in a 55.” The only time I’ve ever gotten the “do you know why I pulled you over?” was the time when I wasn’t doing anything wrong, and I got let go even though he insisted to the end that I was doing 87 in a 70 (white privilege at work).
“Do you know why I pulled you over?” is a trap. It means there’s a good chance the officer doesn’t actually have a good reason to ticket you, and is trying to get you to waive your 5th Amendment rights and incriminate yourself. If you make a guess, that’s a confession of guilt.
But there’s another trap, that I’ve heard of but haven’t yet experienced. It’s “do you know how fast you were going?” With that one, they’re hoping you’ll say no, because then they can name whatever speed they want – you just said you didn’t know how fast you were going, if you deny the speed they name then you’re lying to them.