While this is an afterthought, I just wanted to mention that Amtrak has passenger service between Vancouver and Seattle. The reason I bring this up is that all border formalities are conducted before the train departs Vancouver.
I would suggest that perhaps part of the more intense scrutiny was that you were brining a rental car with (I’m assuming) British Columbia tags over the border.
My brother had the same experience: He was working at a job site in Massena, NY (close to the Canadian border). The closest commercial airport is Ottawa. He flew there rented a car, and came south. Apparently the customs official could not wrap his head around the fact that there was a US citizen coming from Canada to the US in a rental car with Ontario license plates.
