I'm with HarleyEarl on the cell damage part. When flesh freezes, the cytoplasm inside the cells expands and ruptures the cell walls. When it thaws, you lose a good deal of the stucture of the meat and much of the moisture will release into the spaces between the muscle fibers causing the "mushiness" you feel.
This happens more when the freezing process is slow. The faster the meat freezes, the less cell damage you will have.
That being said, it should still be good to eat as long as there's no detectable odor.
Pro Q Frontier, UDS (Little Red), "Turtle Drum" UDS, CG Gasser KCBS CBJ#61160