The remarkable post-pandemic momentum that airlines have enjoyed showed signs of cracks this week, and investors appear to be getting nervous. Shares of Delta Air Lines (NYSE: DAL), Alaska Air Group (NYSE: ALK), American Airlines Group (NASDAQ: AAL), Allegiant Travel (NASDAQ: ALGT), Sun Country Airlines (NASDAQ: SNCY), and Frontier Group (NASDAQ: ULCC) all joined United Airlines (NASDAQ: UAL) in falling more than 10% for the week, according to data provided by S&P Global Market Intelligence, after United issued a first-quarter earnings warning.