Why is this sticker on my car?

There are a number of possible reasons but you probably parked your car in an inconsiderate or illegal manner. You may have commited one of the following offenses:

• You parked across a sidewalk blocking pedestrian traffic. Someone didn't appreciate having to walk around your car.
• You parked in front of someone's driveway trapping their car in their driveway.
• You parked your SUV in a compact parking space blocking other cars in and clogging up the parking lot.
• You double parked.
• You parked too close to someone's car.
• You parked in reserved or handicapped parking.
• You parked in a prohibited space such as a bus stop or in front of a fire hydrant or crosswalk.

Will this sticker damage my car?

No, it should peal off easily. It is a special sticker that is easily removed without leaving behind adhesive.

Isn't it passive aggressive to leave behind a sticker instead of confronting me directly?

Yes.