The FAA airspace plan for 12/26 also included the following:
AFTER 4:00 PM EST LGA/JFK GROUND STOP/DELAY PROGRAM PROBABLE
AFTER 4:00 PM EST EWR/TEB GROUND STOP/DELAY PROGRAM PROBABLE
Based on that information, the weather forecast, and UA's experience flying to and from LGA, they are using their best judgment to estimate how many planes they will be able to get in/out of LGA this PM. Once things go south at NYC airports, they tend to go south pretty quickly.