Typical rotation (i.e. the day prior) appears to be BOS-DCA-JAX-DCA-BOS. Which is to say that the plane flying the DCA-BOS segment usually does a DCA-JAX turn prior. Looks like the inbound from BOS-DCA on the morning of the 12th was also severely and they still needed to do the JAX turn. So they pulled the planes out of their "normal" rotation and got it done.
I don't know if it was a crew or aircraft availability issue that made this swap the smart play, but I'm guessing fewer pax stranded as a result, even if some delayed much more than desirable.