Because market participants are more willing to avoid risks when the index falls than when the index rises. Therefore, when the index falls, the hedging demand for buying put options will increase, and it will also push up the implied volatility of high-priced put options. VIX reflects the views of participants in the options market on the fluctuation of the broader market, so it is often used as a reverse indicator to judge whether the market is long or short.
Extended data
Judging from the past trend, the panic index reflects the market's expectation of future stock market "fluctuation" to some extent. Therefore, when the stock market falls sharply under the influence of bad news, the panic index (that is, the expected volatility) will rise rapidly.
However, if we compare daily, it is impossible to completely correspond to the daily trend of the stock market and the trend of the panic index on that day, because logically speaking, there is no need to completely correspond between the two, so there will be a phenomenon that "the stock market rises and falls and the trend of the panic index sometimes differs greatly".
Baidu encyclopedia -VIX index