Christopher Peel, chief executive officer of London asset-management firm Blacksquare Capital LLP, which launched a new fund of commodities-focused hedge funds in June, says a big contributor to oil’s slide has been momentum trading by computer-driven commodity trading advisors.

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Natural gas was cheaper on a relative basis than crude, but instead of rallying, the sell-off in natural gas accelerated.

Such traders move en masse in and out of commodity markets when prices hit certain thresholds. Sentiment on oil has turned bearish, and “the muscle at the moment is with the trend followers,” says Mr. Peel. Once such computer-driven traders all start to sell, that “by its nature, sucks in the discretionary managers” who also reach a point where they have to sell, if their portfolios have dropped so much that they need to cut their losses.

Another popular energy bet — on natural gas — has hobbled some hedge funds, Mr. Peel said. Several traders recently bet, incorrectly, that natural gas prices would increase, while crude oil would come down, since natural gas has grown far cheaper than oil on an energy-equivalent basis during crude oil’s big runup.

Instead, in July, the opposite happened: while both fuels fell, natural gas dropped 32% while oil fell just 11%, widening the spread between the two fuels rather than narrowing it. Hedge funds who had bought natural-gas futures as part of this bet may have had to unwind them by making offsetting trades to sell natural gas, accelerating the decline of the price of the fuel, he said.