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.
Thats a very good place to start. Why would you even paint a line that looked like a taxi line that wasn't meant to be followed by an aircraft? Unfortunately for the crew they are considered to be the last line of defense and will bear ultimately responsibility for the accident.