One mystery in a dramatic year on Wall Street has been the identity of a trader whose determined acquisitions have sent shares in ViacomCBS Inc., Discovery Inc. and a handful of other companies soaring even when the wider market was down.People familiar with the transaction state that the answer is former Tiger Asia manager Bill Hwang. Late last week Morgan Stanley, Goldman Sachs Group Inc. and Deutsche Bank AG rapidly unloaded big blocks of shares in those companies and others, part of the liquidation of positions at Mr. Hwang’s Archegos Capital Management. The sales approached $30 billion in value, and fueled a 27% plummet on Friday in shares of ViacomCS-an unusually large decline in a broadly held, large-capitalization stock on a day with no relevant company-oriented news. Billions of market value in other companies were wiped out as sales continued, surprising market participants who named the size and speed of these stock sales unprecedented.