People are seen on Wall St. outside the New York Stock Exchange (NYSE) in New York City, March 19, 2021.Brendan McDermid | ReutersWhen investors are stampeding for the exits, it pays to be first out the door.That’s what happened when falling shares in ViacomCBS last week ignited a $20 billion wave of forced selling at the Wall Street banks that cater to Archegos Capital Management, the family office founded by former Tiger Management analyst Bill Hwang.By the time Credit Suisse and Nomura, two prime brokers of Archegos, announced early Monday that they faced losses that could be “highly significant” to the …