As much as you're going to hate to hear it, it is ATC. Been an ongoing problem at EWR due to airport congestion and additional restrictions being placed on the NY area (and increasingly other areas of the country) due to FAA staffing triggers. UA is proactively trimming the EWR schedule to reduce instances of this happening (I believe they just loaded the revised schedule through August). But given basically every day the ATC Operations plans have "N90 - STAFFING TRIGGER, ISLIP AREA 2100-0100Z" it's going to continue to be rough going through EWR for the foreseeable future. For what its worth - many FAA facilities are running staff on a 6 days on 1 day off schedule currently - that's how bad their staffing challenges are, and long term having employees on that schedule is only going to make things worse as they burn out controllers causing them to leave completely.