I believe pigeons are equipped with sensors that can detect objects or humans of which they could do the maximal harm to, and when activated they are programmed to just **** on that particular object or human being.

Pigeons don't "aim". But pigeons and most birds, eat a lot and "poop" a lot, So just happens that usually pigeons are grouped in dense communities flying around and when there lots of pigeons flying, you are in higher risk to be "pooped"