What kind of grass do you have?
Are you mulch mow
Have you ever mowed when the grass was wet or still had dew on it?

I'm thinking the mower was throwing moist grass that did not filter down into the blades of moist grass. Those clumps of grass cut off air circulation and allow disease to develop.

Yep, looks kind of like good old brown patch fungus, but that is quite a sunny slope. If you have been inundated with rain, tho, like we have, it is almost unavoidable. It's been a plague in NC this summer. We water at 3 AM to help, but still have a little -- the first year ever for us. Link below that might be of interest, especially the pics.

Here is a link that might be useful: Pictures of brown patch fungus.

It's bermuda grass. Thanks both for the info.