Could be a fungus.

You mention you laid the sod last year. Did the sod stay really wet (too wet) for a few weeks, or did you keep it just moist on the very top. A lot of times people water too much, and it will develop a fungus that doesn't manifest itself til the next spring if you lay it in the fall.

I imagine it's probably taken root, but see if you can cut a small piece in the dying part to the bottom of the sod layer. If you can see anything resembling mold/fungus, there's your problem. It's still possible you have a fungus that is not visible. You can try a fungicide from a nursery or landscape supply place or it might be best to call an expert this time around.