The possible reason for the decline, according to the analysis: Worry that Powell and the Fed by extension aren't understanding the current landscape.
"Specifically, the equity market likely implies that the Fed is underestimating various risks, and hence is increasing the implied probability of the Fed committing a policy error in the future," Marko Kolanovic, global head of quantitative and derivatives strategy, wrote in the report. "A higher probability of a policy error translates into lower equity prices on the news."
Overall, the market has done quite well this year.
The S&P 500 has risen about 9.6 percent, even though the Fed has increased interest rates three times. Powell has repeatedly said that he believes the economic outlook is strong and that this is a good time for the central bank to normalize policy after years of an ultra-accommodative stance.
However, the market hasn't liked what it has heard specifically from him — and neither has President Donald Trump.
Trump has criticized the Fed on several occasions, a move unusual for a president, saying he is worried the Fed's insistence on raising interest rates could cost the economy the substantial momentum it has built up since the 2016 elections.
The J.P. Morgan paper said there appears to be direct causation between Powell's remarks and stocks because the market had taken a discernible change in direction during the days when the Fed chief spoke.
The bank cited three troublesome statements from Powell: That stocks are overvalued, that multiple rate hikes are needed or necessary, and that a stock market "sell-off warrants attention if sustained." Kolanovic said that implies the Fed doesn't understand market structure and may stay on the sidelines too long.
"If fundamental investors start questioning the cycle, a technically driven sell-off could be more violent and more likely to deliver a knock-out punch to the economic cycle," he wrote. "The new microstructure of financial markets would not leave enough time for the Fed to react."
CNBC has contacted the Fed for comment.
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