I saw this happen in the marathon a lot this year. It was probably compounded by the fact that our corral was like the largest in the history of corrals... lol. My wife and I got there first thing and were up pretty close, but chose to go to the side by the fence. Over time we continued to watch people trample over others to try and get to the very front. In one particular case, I could tell of one person that was going to the front and pretending to be meeting up with (visibly looking for) others. After they got to the front, other friends came and found this person. But they were all acting like they were joining in with an existing group, but not really. They just felt the need to get to the very front. As I suspected, within the first 100 meters, we passed this group. They were walking side by side (maybe 4 across)... and I'm guessing they were "very proud" of the fact that they were successful in getting to the front of the corral.