Good question and looking forward to seeing reply. Could it be along the lines of the yachting registration which start with a K like Gatwick's KK and then Heathrow as the second London airport being LL.
Incidentally, Danish airports run along the logical lines, EKCH-CopenHagen, EKBI-BIllund, EKAH-AarHus and the like. So what do you suppose Aalborg is? EKYT. And why? The RAF code for Aalborg was Yellow Target due to the Gestapo HQ during WW2.
Any-one else got historical relics?