If a troublesome bishop is getting on in years, the Vatican might just decide to wait until he turns 75 rather than go through the hassle of dismissing him. According to canon law, all bishops must offer to resign on their 75th birthday; sometimes the Vatican refuses to accept a bishop's resignation, and he continues to serve either until the Vatican asks him to leave or until he dies. Often, however, the Vatican accepts his resignation once a suitable replacement has been found.
The pope doesn't personally keep tabs on the world's bishops and call them into his office to fire them when they act up. Instead, papal nuncios — like church ambassadors to different countries — keep tabs on their regions' bishops and recommend action to the pope. Whether a bishop is fired or resigns (for age or other reasons), he isn't stripped of his titles. Being appointed a bishop is a sacrament, or a sign of God's grace, which means that no man can undo it. (The exception is when a priest decides to renounce the priesthood — say, so that he can get married — at which point he goes through a process called laicization.)
Historically, bishops haven't always been appointed (and fired) by popes as they are today. Until Pope Gregory VII began challenging the practice in the 11th century, bishops and abbots were sometimes appointed by kings, counts and other laypeople. It wasn't until 1917 that the pope's sole ability to appoint bishops was codified in canon law.
Got a question about today's news? email@example.com. Explainer thanks Christopher Bellitto of Kean University and Thomas Reese of Georgetown University.