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

SoftBank shares tumbled 7.2 percent 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.