I'd pull into a empty parking lot, before I was done there I had to wait in line to check out. I used to joke about it, but everyone was convinced something weird happening.
That's one of the laws of retail: "There are no short lines." If you're not ready to check out, no one else will be either, but the minute you say, even unconsciously, "Well, that's all I need today," everyone else in the store will hustle to the register.
I used to see it back when I was a gas pump jockey in the mid-60s, and have seen it consistently ever since.
About the only place/time the rule doesn't operate is stores within the Unitey States five minutes after the kickoff at Super Bowl; go in then, and you'll be the only person buying stuff.