The cardinal turned 75 years old yesterday, and celebrated with friends and a cake. The resignation letter was submitted because 75 is the mandatory retirement age set by the church.
But just because he's submitted the papers doesn't mean he's going anywhere. Pope Francis, who himself is 87 years old, does not have to accept Cupich's resignation, and can ask the cardinal to continue to serve.
Pope Francis chose Cupich to succeed Cardinal George as the ninth archbishop of the Chicago Archdiocese in 2014. Before then, he had served as a bishop in Spokane, Wash.