I had knee surgery about 5 months ago and now a new MRI shows that I have a menicus root displacement. How can that be? It wasn't there during surger?

Retear. We are at a disadvantage because we were not there at the surgery, but it is not uncommon to retear a meniscus or extend a preexisting tear. The fact that you had surgery presumably for a tear speaks of the underlying weakness of the meniscus. So a new displaced meniscus root tear most likely represents a new tear after the surgery.
Not too unusal. Several things happen with surgery. Some areas are harder to see with arthroscopy than others, and without any indications of additional injury, that area may have been overlooked. Your meniscal root displacement may have been present in the prior study, or may not. You can ask your Doctor to have the two studies compared. Remember that changes in stress after surgery can cause additional injury.

Related Questions

I went to see the surgeon that operated on my knee after I had surgery and a new MRI and it showed a displacement of meniscus root but doctor said no?

Discrepancy. Sometimes surgeons and radiologists lack consistency with their diagnoses. Surgeons have the luxury of evaluating patients clinically. The surgeon may agree with the picture the radiologist is looking at, but your chief complaint may not match your physical exam findings. That is potentially why you see a discrepancy between the two disciplines.