It could be due to a betrayal. If one of them betrayed the other's trust, like revealing a big secret or stealing something important from the other. This would seriously damage the relationship and make it impossible to remain friends.
Perhaps their life goals and directions were just too different. One was aiming for a very stable, family - centered life, while the other was all about traveling the world and having new adventures constantly. They found that they couldn't support or understand each other's choices well enough to be friends.