I started as a cart attendant, so I noticed that cars entering the parking lot would usually correspond with the traffic lights nearby. Pretty simple and straightforward.

What's weirder is that most people exit in groups. Maybe not the same group they came in with. Some people might need to go Guest Services for a return. Some people would stop at Food Ave for a snack. Some used the restaurant. Some would peruse softlines (clothing) for an hour or two with multiple trips to the fitting room. Some would go straight back to the pharmacy for their meds. Some families would come in and split up. And there would be the occasional straggler checking out on their own, but there was an overwhelming tendency for several parties to all check out at once. Then they would form their own traffic leaving the parking lot.

I think some of it is subliminal. Once one group starts to check out, that starts to put the beeps of the register, the hum of the convertor belt, the clatter of items being moved, and the rehearsed conversations of a transaction to float through the air of the front of the store. My hypothesis is that customers shopping in the front who were close to done would have their decision to check out accelerated by this.

Some of it could be the result of all of the other psychological manipulation that happens in retail stores. That is a whole science of its own that I am only vaguely aware exists, but it is very likely that a contributing factor too.

Or it could be emergent from human behavior. For example, studies have shown that people will commute ~1hr max to/from work (I can't remember the exact number, it might have been closer to 45 minutes). Doesn't matter if they are walking, riding a horse, rowing a boat, driving a car, taking a bus, taking a train, or any combination. Once the time starts to exceed that amount, people either look for a new job or new home, and this has had an impact on the way that neighborhoods and cities grow over time.