It doesn't require an event to find 12 people flying from a DL hub to the DC area on a Friday night. Tickets into BWI are also generally cheaper than into IAD or DCA, so it may also be that people willing to pay a fare which gets them a FC seat will fly to a less convenient airport.

If DL's systems showed that it would likely sell all 12 seats in F, it would not sell any in A/P. No different than steerage pricing.