Good question, but it may not be relevant. Maybe they realized they would rather be at the top of a small airline than the bottom of a large airline. 20 years as a captain could make a pretty bad FO. The money really isn't that big of a factor late in a career. It will take 3-5 years at Delta to make up for the earning lost to jump ship. That combined with other factors could have been enough to make the captain want to go back to Endeavour. It could even be a personal issue. Maybe their spouse got cancer and he realized he would rather spend the last few years with more days at home.