One would be indicating that the faucet is being turned on RIGHT NOW (note the -ing): "Why are you turning on the faucet?" The other would indicate that the faucet was turned on in the past: "Why did you turn on the faucet?" So to answer your question, it depends on when the action occurred. Hope this helps! :)

Thanks Andrew!!

If the faucet is already running, the most suited to your situation is the first: Why did you turn on the faucet? Why are you turning on the faucet? Would be used if your friend is about to, or has just begun to turn on the faucet. Why do you turn on the faucet? Would be a very rare, usually rhetorical question that I would only expect to hear from a parent talking to a child or a teacher talking to a student.