It's very possible they just didn't like you and wanted to make your trip miserable. Or they were just hoping to find contraband, mainly drugs. Or they were bored. Or there was no reason. Who knows? From my experience and research, that is SOP for law enforcement in the US. If they can harass (or worse) you, they will.
I'd agree, I got the guys working the Thanksgiving weekend and they were pissed, took it our on me.
I don't even fit into any often searched groups I'm a pasty white Canadian who speaks nothing but English. I'm so common and dull I could be a spy and blend in anywhere.
The guard at the border said "I think you're going to stay with her and not come back but there is nothing I can find that will prevent me from holding you."
I can't see what going through my entire car would accomplish I had one suitcase, which they took into the main building, and nothing else was in the car yet it took them an hour to search it.
Anyway a big FU to the St. Stephen border guards at the crossing in Maine. I honked to them as I drove back into Canada.