robsaw is correct. I can add the following:

I have NEXUS.
I fly to the U.S. every couple of weeks from YYZ.
I was doing OLCI and don't check a bag for my 1-2 day trips.
Things were working perfectly until late August when I started getting called at the gate to show docs and I looked at the screen and it said something like Documents Alert. (Most pax don't see the screen so they don't know what it says)

I did all my bookings on AC.com, then would go to the Advanced Passenger Info online form where you enter passport or Nexus info.
Once you enter, you click SAVE INFORMATION and it is supposed to save it.
Starting mid-August, the NEXUS # was being saved in the PASSPORT field and despite repeated attempts to move the button and re-save, it would result in the same thing.

So for the next trip, I decided to avoid OLCI and try YYZ counter check-in (still no bags).
I see the document alerts message on the screen. Agent takes my NEXUS and re-enters it and poof, good to go

Next trip, thankfully had much more senior agent at check-in. She advised she's had these issues with other pax starting about the same time.

As an experiment, I've used phone agents for a couple of bookings and they also had trouble entering the number in the correct field.

I have spent the past month trying to get to the bottom of it. I know of a few other people who encountered this but also had no idea why.

Here's the thing: At LAX and SFO, the AC agent takes my NEXUS card, runs it through the swiper and BP prints perfectly with all correct info, TSA Pre-Check etc. And it makes a huge difference because at LAX, they now have a test going on with a super-duper speedy line where it takes less than 1 minute from start to finish and you bypass the other lines completely.

So yes, robsaw is correct that U.S. needs to see documents to allow entry and also, AC needs to verify because they are required by the U.S. to do so for every passenger who boards a plane to the U.S. (I've been told by many gate agents that AC could be fined if they do not follow this procedure.)

Apologies for the long post, but in case you are experiencing anything similar, you are not alone and it is not only YVR.

It could be just as simple as yes, they just need to check who you are and docs match, or it could be one of these issues.