
The Justice League disbanded due to disagreements among its members about their future paths. Occasionally, they would split up during peaceful times for rest or when faced with crushing defeats and questioning their impact. Not all splits were equally well-executed though.
Or:
The Justice League parted ways because of ideological differences within the team. Sometimes they chose to separate during tranquil periods for relaxation, other times after facing devastating losses and doubting their significance. However, not every split was handled with equal finesse.