Sometimes the gate agent will page some people for document check, but not all, I am wondering how to avoid this?

The reason is I had 2 bad experiences out of the last few times when I have to interact with AC's ground gate agents for passport checks. (I will caveat by saying both times were at US airports). The less I see an airline employee the better.

The last time (18? months ago) was at DCA when an agent insisted that my I94 showing my TN must be collected. I ended up in secondary again for an hour because of this the next time I went to the US.

This time was at SFO when I presented myself for document check, a luggage tag was promptly printed for my carry on and tagged to my boarding pass, without a word passing. When I inquired if they have the right person, they did (Id'ed by seat number). When I said it fits the regulation size (which I checked using metal grid thing within their sight range, though not sure if they were looking, as they are printing the tag) I was asked to check again. When it fit again, there is not a word of apology for the mistake, not even an acknowledgment. I was accused of attitude when I said they should've talked to me before printing the tag. And some of the people who didn't get the document check (ie didn't have to talk to agent) took aboard some bags that are patently too large.

The other funny thing is that once they checked passport, they still want to see it again at boarding 15 min later.

What I conclude from the experiences is that the less I have to interact with the people, the better off I am. (A truly sad state of affairs for a service industry. But I trust that no one here would object that NA airlines have the worst service by far?) So I am wondering what triggers this document check? And if there is anything I can do myself to lower my chances.

For the SFO flight, I checked in online about 12 hours ahead, had no checked luggage, printed own boarding pass at home, and I had nexus card number entered. I don't remember the DCA experience as much, but I believe it was the second leg of a connecting flight, where I checked in at the counter at origination airport (CMH I think).