United, Northwestern, US, and Delta all went bankrupt in the 2000s. Northwestern, according to Wikipedia, was already experiencing problems due to competition from Southwest. One merger would likely trigger a wave of other mergers even if the airline industry stayed afloat as a whole, as other airlines would have a hard time competing against the new larger airline. This may have been what triggered American's bankruptcy in 2011 (after the market as a whole had recovered), as American had avoided mergers up until that point.