It is thought by many historians that Carloman was coerced into doing it by Pepin. The reason is because shortly after Carloman left Pepin seized the throne from the Merovingians and gained himself a lot of authority. He wouldn't have wanted to split the throne with his brother as that isn't a good move if you want to show yourself as a powerful man. Look at the later Carolingians of you want to see the downsides of splitting the throne. So he needed to get his brother out of the way to make this move.