If the planting hasn't been monitored for watering needs in recent weeks (given our persistent drought this season), then soil moisture stress is the most likely cause. Pachysandra is also vulnerable to Volutella Blight, which can kill stems and cause their collapse. You can inspect the plants for symptoms based on information in the linked page. Management would entail mowing-down affected areas of the planting, raking-out all debris to remove spore-bearing materials and to improve airflow through the plants, and waiting for regrowth. (If the plants have healthy roots that aren't too drought-stressed, they will recover just fine on their own.)

We can't see enough detail in the photo to rule-out Southern Blight, a common fungal pathogen that can kill dense groundcover plantings or other plants, but you can use the information on the linked page to look for its characteristic spores near the soil surface. In our experience, usually declining Pachysandra is afflicted with Volutella instead.

Miri