Wasn't entirely on Boeing, though Boeing did rightly take the fall for it.

Boeing was in advanced negotiations to contribute to the CSeries and, as part of those negotiations, wanted to devalue the CSeries (this was a massive dick move) by having tariffs threatened against it. Of course Boeing and Bombardier would come to an agreement, Boeing would gain the most advanced narrowbody in its class, and finally be able to fill the hole the MD-8x adn 717 had left.

Everyone would be happy.

What Boeing did not expect was a change in US leadership and a president with little to no business sense. The new president used an executive order to put tariffs on the CSeries, which meant Boeing had just taken a massive punch in the face from its own president. If the tariffs existed, not just the threat of them, the CSeries was indeed devalued, but also now had much less value to Boeing.

Since Boeing had been suing for the tariffs (which it fully intended as just a negotiating chip), Boeing took the blame for it, and rightly so. Boeing then also lost the CF-18 Hornet (which it was absolutely going to win) to Lockheed Martin's F-35 - Trudeau had said he saw the F-35 as unnecessary and wanted the Hornet Extension Programme instead, but now his hands were tied, it was politically impossible to go ahead with Boeing after what Boeing had just done to Canada.