Are you watering sufficiently?

My thoughts as well. Sans will do that when they're too dry.

Lescont, the trick to watering a Sans or any other plant with low water requirement is to water (drench) very thoroughly after the plant has had plenty of time to dry out since the last watering. No little sips.

I have it outside and it is very hot (110) So I have been watching the watering closely. It was loving the hot dry temps until about a week ago. the only thing that has changed is we've begun the monsoon season and it has been quite a bit more humid. Could the humidity have caused this? And if so is there anything I can do? FYI I have a swamp cooler so even with the humidity outside the inside is much worse.

Thrip infestations commonly cause curling of sans foliage. Look the plant over carefully for small silverish or gray feeding scars, or areas that appear to be calloused or corky looking as evidence. Also, a scarcity of particular nutrients, or nutrients available in excess can cause curling of the leaves, as can incidental exposure to herbicides (as in spray drift) or other phytotoxins. The best bet is to make sure you have the basics well covered, and you're not asking the plant to grow at the limits of what it's programmed to tolerate. IOW - learn where the sweet spot is - provide it and your troubles will disappear. It's one of the least demanding plants out there.

Al

Humidity doesn't worry them, particularly in warm weather.