There's a chunk of people who will pay to see a movie at a big multiscreen theater, then go from one movie to another and see a whole days worth of movies for the price of one.

As you said, a "weird" movie doesn't keep their attention, so they just distracted themselves with their phones to kill the time until the next movie.

As an aside, the funniest instance of this phenomenon I ever encountered was when I was at an evening screening of an incredibly embarrassing idol anime movie at an AMC and two extremely out of place people walked in partway through, lasted about 5 minutes and then had to bail when one of the singing numbers started lol.