Weather was a factor on Friday throughout the day on the east coast. While the weather didn't get bad in the NYC area until mid-day, United could have cancelled the OP's morning flight preemptively due to weather issues elsewhere.
Two weeks ago, I had a ORD-MEX-LIM-MEX-ORD ticket on AM nested inside a separate LGA-ORD-LGA ticket on UA. Take a guess which ticket had gotten me to the final destination and back faster
