People fear the unknown and can't predict the outcome. People fear failure. People like the current situation. The purpose for change is not clear or relevant. People dislike the person recommending, announcing or implementing the change.
People see change as an attack on their performance and react defensively.
The timing of announcing the change was wrong or perceived as wrong by the people involved in it. People believe that the change will make the boss look good but not me. People have fear of having to work harder because of the change. People have fear of loss of rights or status because of the change. As I've observed people in the middle of the most chaotic change efforts, the change was not being planned or orchestrated by leaders having character, competence and commitment to thinking, saying and doing the right thing.