Some traders noted that the trades were especially suspicious since many of the options expired at the end of the trading session. (financialtrading.com)
Spikes occur whenever liquidity is scarce, for example at the end of a trading session when liquidity providers roll over their activities to the next day. (oanda.com)
Day Order: Orders expire at the end of the trading session 4:00 pm (CT) Monday - Friday for most markets with a few exceptions. (cannontrading.com)