I was curious about the historical precedent here. The earliest usage of "CA" referring to Canada I could find was a 1913 British abbreviation guide, and it doesn't seem to have been in common use until ISO 3166 was published in the 70s. "CA" referring to California was standardized by the USPS in 1907 to avoid confusion with Colorado, and the term seems to have been in wide use well before that point because it's documented in the 1880's Webster dictionary as referring to California.

It doesn't help that the official subdivision code for California according to 3166 is "CA".