Anecdotal, but: I suspect that the CBP have very specific criteria that can flag specific situations. I know someone who would systematically get arrested/interrogated by plain clothes officers if she was returning back to Canada alone. When not alone, no problem. Once, she was on the point of being arrested, when she pointed to her partner, and they stepped back. Although they never wanted to admit why.

After a bit of A/B testing, it seems to have been "older woman traveling alone with a fancy watch" that was flagging her. Without the watch, the problems went away.

CBP is Customs and Border Protection which is the American department for that. Canada has the CBSA - Canadian Border Services Agency. So you wouldn't get harassed by the CBP if you were entering Canada, it would be the CBSA.

sorry yes, I meant the CBSA :)

Same here! When I was on a green card, I got grilled probably 30% of the time going to Canada. We're talking accusations of smuggling a car into the country and harsh questions about exactly what family I was visiting.

Going back to the US (even as a non-citizen!) I usually just got a "Welcome home!".

As an Indian that used to live in Canada for some time, I got the same "Welcome home" with a smile from Canadian immigration. I still miss that as I don't even get that when I actually come home to India.