"Market on close" is an order that someone put in during the day but doesn't want the broker to execute it until the final minutes of the trading day.
This means what it says: there are a lot more buy orders than sell orders sitting out there waiting for the close and the specialist can't make up the difference because it's too big.
buy order imbalance
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Definition A disproportion of aggregate buy orders. A buy order imbalance usually occurs approximately one hour before the market closes when there is late-breaking news that prompts investors to buy in large numbers. When this happens, information on the imbalance is distributed by exchanges and the media in order to try minimize the disparity. In extreme situations however, trading is halted on the specific security in question.
So, the government is getting it's money back on the loan and not letting anyone else that just bought in get out! It can't go up high enough for anyone to want to sell.