Why do frozen greens get fibrous and tough in the microwave?
Microwave ovens cook primarily by spinning water molecules. Frozen water is less susceptible to this affect. I would guess that with certain vegetables, as frozen water turns to a liquid, it is quickly heated to the point that it boils and evaporates. The heating is less even than cooking frozen vegetables in a pot and leads to areas that are somewhat dehydrated and fibrous. 

Try cooking your frozen vegetables in a container with a lose cover and additional water. This will allow them to cook or steam from the the outside in much like in a pot.

http://en.wikipedia.org/wiki/Dielectric_heating