Game theory assumes all agents are rational. They aren't. Pricing popcorn 20% below a competitor may seem like a good idea to a business owner and may indeed sway a consumer to return (people say things like "the ticket is more expensive, but I think the cheaper popcorn is worth it", even though numerically, it isn't)

If there is only some token competition on popcorn prices and there is research showing that that usually doesn't happen in real markets, then that is a pretty strong suggestion they are colluding.