UnitedHealth Group announced a new chief executive Tuesday, a sudden and surprising change following the fatal shooting in December of its UnitedHealthcare subsidiary’s leader. Andrew Witty stepped down from leading UnitedHealth for unspecified “personal reasons,” the company said. Stephen J. Hemsley, who served as chief executive from 2006 to 2017, will return to the role and remain board chairman. Witty will serve as a senior adviser to Hemsley, the company said in a news release. UnitedHealth has been the focus of sharp criticism over the health insurance industry’s practices and has seen its stock plummet in the past year. The Justice Department has investigated its business activities. UnitedHealth’s shares fell more than 17% Tuesday. The stock, which is part of the 30-company Dow Jones Industrial Average, closed at $311.38 a share, well off its recent high of $630.73 in November. The company also said that it has suspended its annual outlook for 2025,…