Unfortunately, we don't see any clear indications in the photos of what caused this die-off other than a likely slow decline of the turf due to the partial shade. Perhaps the soil here is more compacted, poorly-drained, or more inundated with tree roots if a nearby tree has matured. Given your proximity to the Bay, perhaps saltwater inundation or other soil contamination has occurred; it has already impacted some areas of forest along the eastern shore in wildlife refuges, for instance, killing trees and altering habitats. You could have a laboratory soil test performed to see what the nutrient (including sodium) levels currently are.

Since it doesn't look like a site prime for re-establishing lawn, we recommend converting it to a different planting (groundcovers or decorative shrubs/perennials) or a mulched zone over the presumed tree roots. For mulch, you could use either the traditional shredded bark or arborist wood chips.

If you opt to repopulate the area with alternative plants (groundcovers or otherwise), a mixture that includes some native species should provide the most benefit to wildlife. A mixture can also create more seasonal aesthetic interest as plants come into bloom at different times or exhibit different leaf colors and textures. We can suggest candidates if you wish, but would need more information about site conditions (including if deer browsing is a problem) and what size you'd like the plants here to grow (in terms of limits on height or spread).

Miri