While 737MAX is much more established as it is based on the 737 with decades of establishment, the C-Series has begun to show it's capabilities and approval for landing at London City airport (where you can't land many planes, 737s barely can takeoff from there). Swiss has brought in data showing the fuel savings is even better than expected. With these kind of numbers and data, airlines are going to start ordering them. Boeing then feels threatened and now with Trump talking about dumping by Canada felt it is opportune time to try getting them to slap duties on it.