It has been healthy and producing flames regularly for 8 years. The conditions have not changed.
What Causes an Ocotillo’s Leaves to Turn Yellow?
Certified GKH Gardening Expert
Yellowing leaves could indicate watering issues, to much or to little, nutrient concerns and possible insects.
Scale can become an issue for Ocotillos and they are a sucking insect.
The use of an insecticidal soap should clear them up.
Here is a link to refresh you on the care requirements.