Masayoshi Son makes a big options bet and SoftBank investors are worried

Just when investors thought Masayoshi Son was reining in risk at SoftBank Group Corp, the Japanese billionaire’s foray into highly leveraged derivatives is giving them fresh reason to worry.

SoftBank shares tumbled 7.2% on Monday in Tokyo, erasing about $9 billion of market value. The drop came after the conglomerate made massive bets on high-flying technology stocks using equity derivatives -- and despite one report that it has billions in paper gains.

Son’s career has been full of head-scratching acquisitions and strategic shifts, but the 63-year-old had spent much of this year taking investor-friendly steps that made it seem he