I'm not familiar with this particular orchid, but might make two suggestions. too much sunlight, and an environment (rocks) that no orchids that I know of would be comfortable in. That might be places you would look to for a start.

Doing some more reading, it sounds like the sunlight is definitely an issue along with the sand they're planted in. I pulled the rocks back and felt around the base of the plant... don't seem rotted or like they're sitting in water, but just seems like the wrong plant that was installed in this area.

It could also be seasonal. My spathoglotis often wind down a bit during the winter like that. Though, I am a zone north of you. Did the roots look ok when you pulled back the stones?