ToplineShares of Southwest Airlines rose 9% on Monday and headed toward their largest single-day gain in nearly four years, after activist firm Elliott Investment Management announced a nearly $2 billion stake in the company and called to oust the airline’s chief executive and board chair.Southwest operates under “poor leadership” and a business strategy that “no longer succeeds in the ... [+] modern airline industry,” the activist fund said.Newsday via Getty Images Key FactsSouthwest shares rose to $30.25 ...