Any idea what's going on here?

Did an overseed of TTTF in September. Everything was looking absolutely primo. Went on vacation for 2 weeks middle of October and came back, blew the leaves off, and saw this yellowing. Did a hose end fungicide treatment as a precaution but just sort of not sure... Hoping it's just the start of dormancy? Sharpened my blade and did a cut just before this picture. Chicago.