Don’t blame politics for the stock market’s turn south. The stage was set in the options market at the end of last week, says Marko Kolanovic, the widely followed head of derivative and quantitative strategies at J.P. Morgan Chase & Co.

In a Tuesday note, he reminded clients that he had previously warned that the expiration of options could “lift the lid” on subdued market volatility and put downward pressure on stocks.

Friday marked the quarterly “quadruple witching” event, in which options on individual stocks, stock indexes, stock-index futures and single-stock futures simultaneously expire.

After expiration, Kolanovic says the put-call “gamma imbalance” shifted toward puts, which give holders the right but not the obligation to sell the underlying instrument at a set price by a certain time, for the first time in around five months. Calls give the holder the right but not the obligation to buy. Gamma is a measure of derivative price movement.

That shift, Kolanovic writes, left the market “free” to move again and that, “hence, it should not be a surprise that today, for the first time in around five months, we have a meaningful down move and intraday acceleration.”

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The S&P 500 SPX, -1.11% and Dow Jones Industrial Average DJIA, -0.87% both fell more than 1% on Tuesday, the biggest one-day decline for both indexes since October. The declines broke a 109-day streak of settlements without a 1%-or-greater fall.

Kolanovic said the S&P 500 option gamma imbalance turned around $20 billion toward puts on Monday, “significantly contributing to the selling.” He noted there was also a break in short-term, or one-month, momentum that may have inspired “modest equity selling” by commodity trading advisers, who employ momentum-trading strategies. An uptick in realized volatility, meanwhile, is starting to cause outflows from volatility-sensitive investors, he said.

Meanwhile, a pullback by bond yields, which fall as bond prices rise, could reflect short covering by speculators who had placed record bets that bond prices would decline, he said. Falling yields and a flatter yield curve have been blamed for a selloff in financial stocks on Tuesday. Kolanovic, in his note, warned that the bond short covering could inadvertently provide a “’fundamental’ narrative to this largely technical selling.’”

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The market is entering a vulnerable phase, with increased volatility potentially contributing to further equity outflows, said Kolanovic.

And while political headlines flew fast and furious Tuesday over the fate of a Trump-backed health-care plan and France’s presidential race, he maintains the price action Tuesday was “primarily technical and should not be fitted into a political narrative.”