Plastic items are done through a process called injection moulding. This is a process where plastic is melted and injected in a cavity. When the cavity is filled, the mould is then opened to release the product which is ejected from the cavity.
Depending on the plastic used, the temperature for moulding will be different. If the melting temperature is too high, some manufactures will add other materials such as BPA and/or BHT which have a lower melting point.
This is why it’s dangerous when heated or thrown in the dishwasher. The increased temperature will release the contaminants which melts at a lower temperature.