There are several reasons for your "Kwanzan" to have yellow leaves. Water issues, pests and diseases can all cause leaf yellowing.  Since it was recently planted it is possible that the tree is just suffering from transplant shock.  Stress can also cause a tree to go into early dormancy.  It takes time for the roots to reestablish.  Another possible cause is spider mites. They love warm dry weather. These guys can be pretty small and usually you can't see them without some help. In severe cases you may see some webbing. Try a quick check by taking some leaves and shaking them off (or whacking them) onto a piece of white paper.  I there is movement of tiny dots...you have mites. Too much or too little water can cause issues but you would expect to see some browning of the leaves. Nutrient and soil pH can also play a role: however, this tree is pretty tolerant. Check out the spider mite links:
https://extension.umd.edu/hgic/spider-mites-trees-and-shrubs
https://ohioline.osu.edu/factsheet/HYG-2012-11
I hope this helps,