It seems at that time (very late 1980s or very early 1990?), that was what was available. It was originally for British Caledonian. Mandarin got AE, which was originally Air Europa.

The original Corporate Identity was done by a British firm and their original reservation system was supposedly more related to the BA system (all these systems were kind of related, I think they got their own copy, not hosted by BA). Not sure if that had any influence of getting that code.

Y F Chang was supposed to be pretty superstitious - I don't know if he had the 2 letters evaluated by the fortune tellers

Do airlines get to request their preferences (from existing codes) from IATA when they request? Is there a "vanity airline code" fee