For any sell side trader, optimal liquidation is his bread and butter. Given any client order, the trader has to compete against two forces, market impact and volatility risk.
Market impact : If a large trade is executed too rapidly, costs will be incurred as the trades move the market in an adverse situation
Volatility risk : On the other hand, if the trade is executed too slowly, the the position is subject to risk during the time that the shares remain in the portfolio.