I'm not an expert, but I would think that the paranoid types (CBSA / CBP) would find it odd to drive 400 miles round trip for a few hours. Mind you, these are the same types who probably think it strange to fly somewhere just for the miles.

I would think that "normal" behaviour would be to cross the border frequently for business reasons and not so much for personal trips that last a few hours. Since your behaviour falls outside of that, your trips probably just get automatically flagged. But that's just a guess.