So, it could potentially be the reverse of what I suggested then (spoke to hub, rather than hub to spoke)... maybe there was a potential for something to go wrong with the earlier DCA-JFK flights, so later flights were being held for IROPS reaccommodation to reduce risk of having to reaccommodate passengers at DCA or JFK if they would miss the last INTL or transcon flights out of JFK that evening.

Pure speculation obviously, but as I mentioned, at least on other airlines, I have seen revenue management utilize this type of inventory control for flights later in the day.

The other time you'll see this is when coach is oversold, so rather than zeroing out coach, they'll open up coach inventory at the expense of premium cabin inventory (presumably because it's easier to sell - changing J3Y0 to J0Y3 may result in additional last-minute revenue), so they have to protect seats in the premium cabin(s) to make sure the entire flight across all cabins does not become too overbooked.