“When some of these big moves happen, people thirst for a rational and logical explanation, and indexes and ETFs often fit that bill very nicely,” Lance Humphrey, a money manager in the global multi-assets team at USAA Asset Management, told Bloomberg. “Indexes and ETFs are very easy punching bags.”

For example, major indexers made the decision to shift Facebook and Alphabet into the newly created communications services sector and removed the traditional tech names from broad technology sector-related indices.

“The stocks that were sold were also very volatile, which means if there wasn’t a concurrent buy as part of the rebalance, then they would probably move a lot more than average,” Oliver Brennan, a macro strategist at TS Lombard, said in a note. “I’m not trying to suggest the trigger and the sole cause for the selloff was the rebalance, but it happened at the exact worst time.”

TS Lombard calculated that sector ETFs sold around $10 billion in Facebook, Alphabet and other former tech-related stocks.

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