Suspect somebody got a Margin Call and had their positions liquidated. The time period between 17:30 - 18:00 EST is typically set aside for daily maintenance by CME and most Brokers. Between 16:15 - 16:30 EST the ES market is closed and accounts settled. 16:30 - 17:30 ES opens but this period largely is used to settle accounts. On volatile days having a few orders on the books at the edge of the no bust zone during this "settling" period may catch you a few bargain fills from those not fortunate to live to trade another day.