You can guess the answer to the question, but you really don’t know with any certainty why the cop made the decision to pull you over. Maybe you were going 5 MPH over the speed limit but you were pulled over because the officer saw that your rear license plate – unbeknownst to you – had fallen off. Maybe you were Driving While Black. Maybe it’s the end of the month and the officer needs to make her ticket quota. Maybe he’s having a bad day and pulls you over when he wouldn’t otherwise bother.
Whatever the reason, the cop does know. Which is why it’s a BS question to begin with.