I was thinking your septic bed because it's so squareut usually the grass is always thicker and greener on there. Did you pour anything caustic down your toilet or sinks that could have cause some bubbling and affected the grass? Can't be cinch bugs, they don't cut a nice square patch like that. Does make you think that maybe something hovered just on top of the spot...

Spooky for sure but there has to be an answer.