Answer
José Gervasio Artigas and Estanislao López were posing a potential threat to Buenos Aires that Belgrano had to move his army. They were the leaders of the provinces of Entre Ríos and Corrientes, respectively, and they were both opposed to the rule of Buenos Aires. Belgrano was forced to move his army to the north to deal with this threat, and this allowed the Spanish forces to retake Montevideo.