It sounds like you are following good lawn care practices in general. Grubs could have been consuming the grass roots. They might be below the surface of the soil now, though, since they do work themselves deeper into the soil layers as we approach winter as protection against the cold. Brown Patch is another common turfgrass ailment that causes summer die-off of patches in lawns, with susceptibility varying between grass types. Weather and irrigation play a large role in how vulnerable turf is to disease and how easily those diseases spread in any given year. We also saw widespread infestations and heavy damage in some areas from Fall Armyworm, but no insecticide intervention is useful at this time for those.

We suggest that you renovate or at least overseed the browned sections of the lawn. You could comb off the dead grass with a stiff-tined rake and then put down grass seed or sod, amending the underlying soil with compost if the soil is compacted. Another option is to core-aerate the area and overseed. The aeration process will help the seed contact the soil amid the other plant debris in order to improve germination. However you do it, you need to get the seeds in good direct contact with the soil.

We are now at about the deadline for overseeding cool-season lawns, but you could still do it this weekend. You have a longer window of opportunity to repair the areas with sod if you want to do it that way. In either case, both would need regular and careful attention to irrigation so the new grass establishes well, especially since our autumn rains can be unreliable and it has been dry lately.

Monitor for grubs in about July-August of next year, which is when they are higher the soil profile. Grubs do not typically affect turf-type tall fescue lawns. Grasses most susceptible to grub damage are bluegrass and creeping red fescue. Research shows that damage will occur when grub populations exceed 6-10 grubs per square foot on bluegrass, rye, and fine fescue lawns and 8-15 grubs per square foot on tall fescue lawns, so the best thing to do is to monitor for them early. There are a variety of control options available, outlined here: https://extension.umd.edu/resource/white-grub-management-lawns

Christa