My wife and I call them snugglers - a term which came down to us from Yorkshire. You park your car in an almost empty car park, but when you return it is quite common to find a snuggler has parked next to you in spite of all the empty spaces. Why is that?
One possibility is that some people use another car to guide them into the parking bay. They can't see the lines and don't have the spatial awareness to park without lining themselves up with something visible such as your car.
Are there other possibilities?