There is probably nothing wrong with your sago. They do this every year, the bottom most fronds die back and the new flush of growth comes from the top. You can just trim those off. This is how your plant makes a trunk. They are very slow growing. Some people cut ALL the fronds off of their plants in early spring and let the new flush come out. Other people leave the 'skirt' on. I do both, the one I leave the skirt on is the cover for an armadillo who has a hole under its shelter. I don;t want to expose him so I leave it. The others we have, I trim the bottom dead fronds off every summer after they have finished jettisoning all the fronds they want to get rid of.