I'm pretty sure it really is random. Of my 15 YYZ-USA flights in the last year, I've gotten the random check 2 or 3 times, never by the person who looked at my boarding pass. As a business traveler, I've often bought last minute, one-way tickets. Never gotten the dreaded "SSSS" printed on the BP, and it seems to not affect whether I get picked for the secondary or not (unlike intra-US flights, where it definitely increases the chances).

My experience has been the same: they do a quick look in the carry-on bags, with the explosives swab, plus (sometimes) a brief pat-down/wand secondary metal screening.