It's a combination of heat, UV irradiation, and hydrolysis that happens within a microwave and affects said plastic containers.
However the article refers towards polypropylene as the main cause of concern, due to its amorphous nature, it tends to break into microplastics and nanoplastics.
The earlier tends to be discarded by our kidneys (at a cost), the latter can get through membranes due its size and cause different types of trouble.