The most likely reason for the leaf yellowing is drought stress (water deficit), especially if you are having the heat wave I hear about in Texas this summer. A newly planted tree does not have an extensive root system yet to seek water from a wide area, so it's up to you to water adequately this first season and next while it gets established.
Water deeply three times a week during hot season and once a week after that, and water the entire area where the hole was dug for the tree planting. The first time you soak it good (today because it's urgent), when you get it muddy wet, use a stick or a shovel poked straight down all around the planting hole to be sure all the soil was settled during the planting and there are no air pockets where roots can dry out.
Mulch the soil surface to help retain soil moisture.