Traders like Michael Pomada help explain why the stock market is going through its most turbulent period in recent memory.

Mr. Pomada was in good spirits as he drove his convertible to his office in Los Angeles’s Century City complex before sunrise on March 9. Investment funds managed by his $4.5 billion firm, Crabel Capital Management, were up about 5% for the year. He wasn’t especially concerned about financial markets or the economy, even though oil prices were tumbling that morning.

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