Improper sod installation due to build timing and weather.
The sod is too thin from using a tactor sod layer. It stretches, pulls and becomes thinner as it is being worked. Often the sod is purchased and sits on the site until the prep work is done......then it rains on the sod and the silt erodes away exposing roots. As you install the sod, it will be thin in places and thicker in others. Contractor's will not throw away or trim out thin spots........this costs time and money. Bermuda will fill in and thicken if the turfgrass is kept, watered, fertilized on schedules.