It is hard to say from the image alone. December is not the best month for planting so it may not have had a chance to get well established. My guess is that it may be more related to the root system, watering, and lack of root development since it was doing well until the real heat hit. Digging into the soil next to the root ball to see how wet or not wet the root ball and soil are, might tell you a little bit. Something we often see is it is planted too deep, and the soil does not drain well. The top of the root ball should be right at soil grade or slightly above. If you pull the mulch and soil away from the top of the root ball and have to dig several inches to find the top of the root ball, it was planted too deep. You might reach out to the landscaper to see what kind of warranty they offer. Additional assistance can also be found at the Rogers County OSU Extension Center - https://extension.okstate.edu/county/rogers/.