More detailed information and graphics now becoming available. Evidently initial story stating that conflicts were south of the airport by Mount Vernon was incorrect. Arriving plane was approaching from the northwest, and departures were from RW 1 to the north.
Given the tight vectoring of Brickyard 3329 (i.e., not starting his approach along the Potomac from Cabin John), it appears that he was anticipating an arrival on the slightly diagonal RW 15 rather than the reciprocal RW 19.
Congress, agencies investigating incident at Reagan National Airport - The Washington Post