I've made the same trip many times, although not recently (exchange rate isn't very favorable). (In fact, such day trips are also common across the southern border, IME).

I do find the questioning unusual, IME. I know many people who go to YVR from SEA for a long day trip - shop, bit of sight-seeing, good meal and a late drive (after traffic finally eases up) home. Sounds like you may have been flagged for some reason. That kind of questioning would be unusual (not unheard of, but not usual) at the regular lanes (can't even imagine the backups if they question every car/pax to that degree).

There are also lots of YVR folks (exchange rate now favorable to them) who regularly go south of the border on daytrips, often Costco/gas runs.

The regular use (if they are tracking it) of a rental car might raise an eyebrow if you're a leisure traveler, although your explanation makes perfect sense to me because I do the same thing.

It's a nuisance, I've had some real jerk agents (especially US NEXUS lane agents - I've almost been tempted to take the regular lane back when it is short enough). Just be glad they don't appear to have profiled you as a possible drug mule - if they had, they might have torn the car apart.