As Janet Yellen testified before the Senate Banking Committee, gold sank 1 percent in 10 minutes, taking the metal back below $1,300 for the first time in nearly a month. But rather than being the victim of a single massive bearish trade, it appears the metal was reacting to a slightly less dovish outlook from the Fed chair than some gold holders were expecting or hoping for.

"It's simple," said gold expert George Gero of RBC Capital Markets. "Yellen was a little bit more hawkish than expected. That signals higher rates sooner rather than later. And that's anti-inflationary, and presents competition for gold."

After all, gold does not bear any yield. So as yields on fellow safe-haven assets like Treasurys rise, gold becomes less attractive.

Still, several traders noted the oddity of gold moving so quickly on comments that didn't surprise many people. At 10:55 a.m. EDT, about 7,600 gold contract traded, which means that nearly $1 billion in nominal gold value changed hands in that minute.

This boost in volume led to speculation that gold futures fell because someone "dumped" $2.3 billion worth of the futures. But market data expert Eric Hunsader of Nanex says that doesn't comport with what he's seeing.

"I don't see anything unusual like that," Hunsader told CNBC.com.

For Chicago-based trader Jim Iuorio, the explanation could be a bit more nuanced. He believes that Yellen's comments caused one trader to sell gold, and that this sale triggered stops around the $1,300 level.

"It was a whole lot of volume all at once, and Yellen didn't really tell us anything new," he said. "I think one guy was waiting for a reason not to sell, and when Yellen didn't give it to him, he sold—which led to a lot of other orders getting triggered."

—By CNBC's Alex Rosenberg.

Watch "Futures Now" Tuesdays & Thursdays 1 p.m. ET exclusively on FuturesNow.CNBC.com!