Betting against bitcoin was popular, at least a few weeks ago. It was a “crowded short,” explained Dave Weisberger, CEO of CoinRoutes, an order-routing service for cryptocurrency trading. But when the market didn’t dive as expected, traders with short positions rushed to cover themselves, buying indiscriminately in bitcoin’s spot market. “Short covering is often price insensitive,” remarked Weisberger.

As the short sellers bought up bitcoin, it helped send the coin soaring. The theory explains the price bump, and it also suggests the recent surge may not be long lived.

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