Flew that a few times. Helicopter route 4 is on the east side of the river. Most airliners land 01/19, which keeps them on the West side and no conflict. Landing 33 crosses helicopter route 4 on short final.
From ATC audio, PAT25 called vis sep prior to the mid-air. Speculation, but they may have spotted one of the other airliners that usually stack up along the Potomac in sequence, or lost visual amongst the background lights (not sure if 12AB normally runs on NVGs there...the city gets very bright so it was often a coin toss on whether to scan mostly on or off goggles.)
(I was once told to orbit over Bolling for landing traffic 33, at night.)
A bit of a nightmare situation.