pthalates and BPA, both proven carcinogens, are both ingredients of food-grade, microwave-safe plastic food storage containers (same source). Experiments done in the 1990's that even at room temperature, plastics aren't inert — they leach into the materials they contain in tiny amounts, but enough to act as hormone mimics. If you heat such a food container, the microwave radiation makes the molecules of the container move require heats all the molecules it touches, carcinogenic plastic included.
Even the snopes.com article "Plastic-Tac-Toc," used to refute the idea that plastics leach carcinogenic chemicals when heated has this quote from Dr. Rolf Halden of the Johns Hopkins Bloomberg School of Public Health:
In general, whenever you heat something you increase the likelihood of pulling chemicals out. Chemicals can be released
from plastic packaging materials like the kinds used in some microwave
meals. If you are cooking with plastics or using plastic utensils, the
best thing to do is to follow the directions and only use plastics
that are specifically meant for cooking. Inert containers are best —
for example, heat-resistant glass, ceramics and good old stainless
Why Halden failed to note that plastics that are specifically meant for cooking include carcinogens is anybody's guess.
Get rid of plastic food containers, and especially don't re-heat foods they contain in the microwave.