There are many different reasons, variables, and priorities to determine which flights get delayed and which ones do not. What happens one day will be different than a different day. Actually several different departments will work together to communicate what is going on. I am sure this happens at all major airlines like AA and DL not just UAL. Just some examples.
Yes small planes to close in cities will usually get cancelled first. That is why for say Chicago you will see express flights to MKE, MSN, SBN get cancelled or delayed as it is close by and there are other transportation options like car, bus, and train.
It depends on the plane routing. For Example if EWR is on flow and say there is a 757 in MCO and a 737 in FLL both going to EWR at around the same time but the 757 will then go to Europe while the 737 will go to say BOS the priority might be get the 757 out first as to not impact the EWR to Europe flight.
What is the crew doing. If you have pilots on one flight high on duty time and might go over and flight would cancel then the priority would be get that flight out. So you have a plane in ATL going to ORD with a crew high on time vs a plane in RSW with a crew fresh from hotel the RSW flight will get delayed.
What type of passengers are on it. Are there numerous GS or 1K passengers on it. Are there groups of people going to hard to reach destinations or passengers going to other destinations where the next flights are full. For example is there a large group of 8th graders in DCA for their field trip touring the capitol going DCA ORD then to a small under served city like in North or South Dakota. If they misconnect in ORD how do you get a group of say 30 kids and parents if the next flights are 50 seat RJs.
Unfortunately there is no set answer and that is why in the command centers or I think United calls it their SOC or NOC various people look at the flights to see what has the least impact on the entire system.