Tencent Holdings Ltd.’s sell-off may get a lot worse after the shares failed to hold above their key support level.

Asia’s biggest stock closed down 0.3% in Hong Kong on Thursday, despite an otherwise upbeat market in the city. Tencent is now trading below the key level of HK$320 that supported its shares on three occasions this year. The stock has lost about 20% since a peak in April, equivalent to some $93 billion in market value.

While Tencent has been stuck in a downtrend for months, selling was particularly aggressive earlier this week despite no apparent trigger. Theories circulating round some trading floors included souring sentiment from investors in China, as well as concern that Tencent’s decision to air National Basketball Association games may backfire.

Adding to jitters this week was a local media report that China is considering revising a law to control young people’s online gaming activities -- a business that remains one of Tencent’s most profitable. The Internet giant will report third quarter earnings on Nov. 13.