Airline agents and other airport workers wryly describe flyers who crowd gates waiting to board as “gate lice,” a none-too-appreciative term for an unappreciated behavior. Pushing and squeezing to get on a flight as quickly as possible often results in just making the whole boarding process slower.

So why do people do it, especially if they have assigned seats? Psychology experts offer two explanations: conformity and competition.

First, humans use other humans as sources of information and behavioral cues. If others are moving, you may think it’s time to move too. “People will do any weird thing if they think that’s the way to behave,” Shira Gabriel, a psychology professor at the University of Buffalo told The Washington Post. “When you see people lining up, getting ready, it makes you feel there’s a benefit to that.”

That’s conformity. Competition arises when you see others moving and believe that not doing the same will bring negative consequences, like losing the last open space in the overhead bins or having to check luggage.