The term structure of volatility inverts typically when there is heavy demand for the front-month option markets. Kind of like what I'm sure happened during the Brexit event last year in that many market participants, in the week preceding the announcement, went into the S&P 500 options market to buy insurance should there be a strong negative price reaction to whatever the news ends up being. Lots of demand for option protection in front-month = inflated front-month option prices compared to relatively lower prices for further dated contracts = high implied volatility compared to back-month (further dated) contracts = option implied volatility term structure inversion.