There is, on most exchanges, an auction at the end and start of the trading session. This is meant to clear outstanding orders that have not been executed that trading day (for the closing) or orders that were placed outside of market hours (for the opening). This is also a chance for the traders to build and clear their day's trading books and put themselves into their correct position for the day's trading or to sit overnight. It being thanksgiving in the US today it is likely that they wanted to take very specific positions to hold for some time until market conditions re-normalize after the holiday.
The closing auction is the most important one for big institutional traders and market makers who need to recover their desired position to be held overnight having moved away from this position during the day's trading. Market makers are paid to use their holdings against their own interests to increase the liquidity in the market. This sometimes extends as far as going from a positive position to a negative one during the trading day. At the end of the day they need to balance out their positions and return to the position that the firm has decided they want to hold over the long term so that they can return to their own interests. To do this they need to furiously trade at the closing auction so there can be a lot more movement at close. Day traders, and anyone else who doesn't want their trades to settle, will also be looking to zero their positions at close as they don't want to incur the costs of holding a position to delivery.
There are also order types that state that they must trade at close or must be filled as close to VWAP (volume weighted average price) as possible. VWAP and guaranteed VWAP orders that don't get filled completely during the rest of the session need to be filled by the end of the day so result in a marked increase in trading where the price has been moving in a direction that would improve VWAP for the trade. Traders will again be rushing to fill these in the auction.
Looking at today's date and assuming that you looked at this data in the last day or so we are also around the options expiration dates for the various exchanges so it is likely that the institutional traders were trading into a position to cover any options expiring this week.