@graydon But do you think anyone would come up with the idea of abbreviating "Canadian" CDN, if it wasn't already a letter combination they'd been exposed to their whole life? Sounds to me like the CDN existed first and then people came up with a meaning to match.
No other country writes an abbreviated adjective/possessive for their country in their hashtags, it's just ukpol, uspol, sepol, hkpol, fipol, etc.