See the photo in post #29.
The taxiway onto which the 737 was turning (leading to Kilo, Lima and the runways) is 200 feet wide (between the edge markings) and has not one, but 3, parallel taxiway lines which allow two narrow-bodies to pass each other safely, provided each keeps to its own side of the taxiway.
It seems likely that the unfortunate combination of the "keep left" imperative and the ambiguous edge markings led to inadvertently following the LH edge marking line while thinking that it was one of the 3 prescribed taxiway lines.