Hot days take the moister out of the lawn at different rates. And when you have subterranean features like the septic or even boulders and rocks to close to the surface it can do this. I gained a lot of respect and good will from a client once who's front yard was 100% sodded and every year he had reoccurring dead spots in the same place. He asked what we did I told him sorry you got a rock close to the surface there under that sod, the guys who in stalled it did a half ass job. He shook his head in disbelief and walked inside next week there's a huge pit where that spot was and he goes you wouldn't believe the rock that was there touching the bottom of the sod. Not much you can do about it other then digging it up to find out whats wrong and that's not worth it.