“Do you know why I pulled you over?” Translates from cop speak as “Are you stupid enough to incriminate yourself for my benefit?”
Never answer this even if it is obvious. At best, you have just admitted guilt. A worse possibility is that you confess something else:
You: “Because I didn’t come to a complete stop / speeding / seatbelt?”
Cop: “No, your tail light is out. But I’ll write that down too.”