Just wondering…but why would a 757 takeoff from JFK only to land at LaGuardia a few minutes later?
AMERICAN AIRLINES 9636- just a few minutes ago
Positioning flight. American Airlines flights in the 9xxx series are ferry (positioning), charter, or maintenance flights. This hold true for many domestic airlines.
Take a look at the other flights of AAL9636. Many of them depart from Tulsa. This is a major maintenance base for American.
Now boarding flights 9636 from our hub in JFK to our focus city at LGA. Even though you will have saved time by just driving, you will get the benefit of flying in our cramed cabins, and be in our planes which will most likely be late. Thanks for flying American. Laugh at those cars as they pass you en route to JFK. But rest assured that you will the get the 500 mile minimum.
Yeah, Dami is right 
. It is a positioning flight for many possible reasons. Although I do remember I believe United doing some marketing flight between Chicago Ohare and Midway a few years ago.