Some people do it because they want to secure overhead bin space, some seem to do it because on flights Zone 1 or elite boarding can be a huge chunk of the plane and they want to get ahead of the rush.

People who crowd the boarding area before their zone is called are often humorously referred to as "gate lice" by frequent flyers. This behavior is primarily driven by "overhead bin anxiety." As airlines have increased fees for checked luggage, more passengers carry larger bags onto the plane. Because overhead bin space is a finite resource, travelers fear that if they are among the last to board, they will be forced to gate-check their bag, leading to delays at their destination. There is also a psychological "herd mentality" at play; when one person stands up, others feel a competitive urge to follow suit to avoid being "left behind." Additionally, some passengers find the gate seating areas uncomfortable or insufficient and prefer to stand to stretch their legs before a long flight. While it can cause congestion and frustration for those in priority boarding groups, the root cause is almost always the competition for storage space inside the cabin.