Were the later LGA flights full? They offer pretty frequent service to MCO from LGA. They've also redone Concourse A so it's actually a nicer area to wait now than it used to be.
In any event, I thought they kept an extra A320 at LGA the way they do with the A321 at LAX. I've seen them sitting at a hard stand on occasion. But maybe they were just getting maintenance.
Too bad they couldn't just ferry one over from JFK. Must not have been a full flight.