May also happens to be the month construction began on one of EWR’s two commonly used runways (though they do have a smaller third runway). This severely reduced the amount of traffic the airport could handle and EWR attempted to keep operating the same amount of scheduled flights as usual, it was a real mess.
runway construction is only part of the story, i think. in May, there was a number of complete ATC meltdowns causing ground stops. if you look at the stats, the majority of delays at newark in May are attributed to ATC.
granted this was an especially egregious situation and not the norm, but it feels like these types of issues are on the rise based on my anecdotal experience. there were a number of full ground stops at newark due to ATC in the weeks after this. it was national news.