It is caused by the change of the stress area after deformation.
Generally speaking, whether a component will break is defined based on the yield strength, but usually a certain safety factor needs to be added during the design process, which is the role of allowable stress. Of course, the amount of deformation depends on the size of the model. For example, if the span is very large, it will deform greatly under the action of gravity. Therefore, too much deformation is unacceptable. If it is loaded for a long time, it will definitely cause fatigue permanent deformation. In addition, you need to check whether your model and constraints are correct. I don't know how big your model is. If you calculate such a large deformation, I think there is something wrong with the design. Please check it carefully.