Crowd surges are becoming more frequent at large social gatherings, such as festivals, sports games and holiday celebrations. After the latest deadly crowd surge in the Itaewon neighborhood of Seoul that claimed more than 150 lives, many are left wondering what actually causes a crowd surge and how you can avoid one. Keith Still, a crowd safety expert and visiting professor of crowd science at the University of Suffolk in England, joined The World’s host Marco Werman to discuss.