Maybe United is running some sort of sale? I checked my flight for later this year on American and the price is slightly higher than what I booked it in March. Various airlines seem to randomly have sales to select cities. I assume it is based on some algorithm they use that shows lower booking for certain cities and future dates.
Be wary if you’re connecting through Newark (United has many flights there). Newark has had many issues. I had extended family traveling through there and it was a mess. It’s been in the news as well. Issues here caused issues at other airports.
The price on our flights on United went down and we saved a little over $200 on our 2 tickets. I was checking to see how full our flight was so happy I checked!
I checked again last night to see how full our flight is leaving this weekend and the price increased $250 per person. Yikes! Like you said not not a sale but the price drop was nice while it lasted.