You are likely flagged. The US has pretty lax security on credit card transactions (I mean, Canada had Chip+Pin about a decade or so ahead of the USA.) On top of that, a lot of institutions are getting hacked lately. When they think they've been hacked, they'll just flag every account holder and have computers double-check everything, rejecting anything suspicious and then (maybe) getting in touch with you. But if they're not certain that they were hacked, best to save face and not send out a notice.
Seems like for whatever reason, they are being extra stringent with your transactions.
I have ordered from Amazon for years now up in Canada. They're really nice. Direct shipping from them is incredibly fast, they're one of the only companies that delivers on weekends (everyone else is M-F), they offer free return shipping, and often their prices are the best by more than 5%. I don't have anything bad to say about them, other than their product search sucks. You're relegated to looking through stuff page by page. Also, most Amazon sellers don't bother to offer their products in Canada, or just use placeholder shipping methods. It's very common to see something like one of those fridges say $1238.54 CAD + $354.80 shipping. Absolutely insane pricing on products warehoused in the US.