Check out how some airports have gotten their codes.

The site claims that "Airport codes starting with ‘Y’ designate Canadian airports." Not necessarily true - just a few examples of non-Canadian airports with IATA 'Y' codes include Yakutat (IATA: YAK; ICAO: PAYA), Yakima (IATA: YKM; ICAO: KYKM), Yakutsk (IATA: YKS; ICAO: UEEE), and Yuma (IATA: YUM; ICAO: KNYL).