What Is Triple Witching?
Triple witching is the simultaneous expiration of stock options, stock index futures, and stock index options contracts all on the same trading day. This happens four times a year: on the third Friday of March, June, September, and December. A common expiration date for the three types of equities derivatives can cause increased trading volume and unusual price action in the underlying assets.
Short sellers use options and other derivatives to short the stock, when the options are about to expire they dial up the manipulation of the underlying stock to make sure their options are in the money.