This has always fascinated me, mostly the last two summers sitting in the lounge or on the tarmac at EWR. I've always figured that with UA it is some form of gate Tetris, with incoming international flights taking precedence and things falling from there.

Does UA have an algorithm where they determine the amount of connections on an incoming domestic, do they prioritize the use of a specific aircraft in continuing operations after it lands, how much are they using specific crew times and locations as a way of determining which flights move, etc,?

At EWR, I feel A opening has helped let the air out of the balloon ever so slightly, but I still think at EWR (and ORD, etc.) that cumulative delays are in very large part made up of simple lack of gate access once the weather begins to clear. I know there are MILLIONS of reasons it can't happen (runways, capacity concerns, gate connectivity, crew, etc.) but always wished that, instead of 5 or 6 daily flights to places like BNA and RDU, UA simply flew two heavies per day and saved stress on the airports.

In OP's example, I would guess that the answer is quite possibly simple....DL almost certainly would have a gate for the ATL arriving flight, whereas UA would not.