I am a US citizen who has left the States a small handful of times. I received probing questions twice while crossing the USA-CAN border (which I have crossed ~6 times), once where the agent got angry with me because apparently I wasn't answering/following her directions well enough, and the other time the agent seemed to just be following a protocol. The only other time I was questioned was on my way to Iran, but everyone was getting questioned. That flight was delayed by over 3 hours too.

I do not look like an immigrant or other marginalized group that stereotypically gets extra attention at borders. I know the difference, because my (American) brother-in-law gets 'randomly selected' everywhere for the misfortune of having Arabic ancestry.