A sell-off is the rapid selling of securities such as stocks, bonds, ETFs, commodities or currencies. A sell-off may occur for many reasons, such as the sell-off of a company's stock after a disappointing earnings report, the departure of a important executive or the failure of an important product. Markets and stock indexes can also sell-off when interest rates rise or oil prices surge, causing increased fear about the energy costs that companies will face. Sell-offs can also be caused by political events, or terrorist acts.
Source : Investopedia