I was sufficiently curious about this that I went and checked AirAsia's Wayback machine for this. AirAsia's flights moved around 1st of Oct 2012 from BKK to DMK. For this date, it shows two Bangkok options, listed as "Bangkok (DMK)" and "Bangkok (BKK)"[0]. By January 2013, only DMK is shown, as "Bangkok (DMK)"[1]. By the next year, it's labelled as "Bangkok - Don Mueang")[2].