What causes plants to get root bound before setting in the garden?

This is my 1st year I have had my own small ( 8 X 8 ) green house. My plants grew very well but they became root bond. I used miracle grow potting soil in styrofoam cups with a small hole , the size of a nail,in the bottom for drainage.In spite of this I prepared my garden & set these plants out.Then it started raining just in time.I usually have a good garden when I buy my plants but I've decided to grow my own from heirloom seeds because I feel uncomfortable having to depending on others to supply my food.I have begun using heirloom seeds because they have not been tampered with (another reason) thereby increasing our dependency on others to provide our food.I have asparagus,potatoes,onions.sweet sugar snap peas,corn,green beans,cantaloupes,water melons,romaine lettuce,3 different kinds of cabbage,okra,cucumbers,& squash,later plan to add tomatoes & i'm sure i'll think up some other stuff to put in it.And I fertilize it with horse manure.