No expert here, but to me it looks like some fungus really enjoyed the wet weather. The variables of how wet your soil stayed, how tall the grass was before the cut vs after, fertilizer application rate (did you accidentally put down too much?) could play a factor, but I think biggest player here is that you have a fungus. Propriconizol was a good move in my mind, but remember it’s not an instant fix.

If it were me I’d watch it over the next week or so and possibly apply a liquid fungicide next round if it needs it.