Don't you think that grass growing in a corner, surrounded on 2 sides by a
6 ft wall or fence might be weakened by bad light, and this coupled with
the mower scraping the top off as it's turning the corner may have more to
do with it than bad drainage?

I considered this, but in two or three of the corners, the light isn't
actually bad. It gets decent light for two-thirds of the day. They
weren't areas where the grass was ever particularly scalped either. It
still looks to me like these worm-casts are taking over. Maybe I can try
and take and post a link to a photo, if that would help.

Neil