Im not familiar with goji, but I'm thinking transplant shock. Did the plant seem
root bound when planted?
If so it could be stressed from the moisture issues, they can dry out quickly this time of year.
My search brought up this-
"Other Causes for a Plant with Purple Leaves
If you have a plant with purple leaves, it may also be due to elevated levels of anthocyanin, which is a purple colored pigment. This pigment builds up when a plant becomes stressed and normal plant functions are interrupted. This problem can be very hard to diagnose as other factors can cause the pigment buildup such as cool temperatures, disease and drought."
A phosphorous deficiency was also mentioned, but I'm leaning toward stress.