A lot of good points here, but I'd also like to bring forward another hypothesis which partially explain the incredible speed at which this is moving forward in the last few months (even though things have been brewing back and forth for years, decades).

The US has become a hostile state. For Five Eyes, Six Eyes, Nine Eyes, and Forty Eyes that means much less domestic intel since all the Eyes were sharing domestic intelligence to circumvent stronger protections on their own citizens. Canada would spy for the US and the UK, and vice versa, which was a neat way of getting rid of pesky rights afforded to citizens.