The volatility of the equity markets are subject to seasonal influences. Thus price swings during the summer and at the turn of the year tend to be diminished. As yet, participants in the options markets have hardly noticed this at all.
Please note that the stock market crash in October 1978 strongly influenced the average. For mathematical reasons the influence of single extreme instances is very high, stronger as in other seasonal charts. This is based on the method of calculation which uses absolute values. The high October and November values should be observed with this aspect in mind.