The situation that would most likely decrease the carrying capacity for humans in a certain region is a crop disease becoming resistant to medicine.
Explanation:
If a crop becomes resistant to medicine, it means that the crops will become immune to the medicines that were used previously on the crops to treat the disease. As a result, becoming resistant to the disease would mean that it will still remain in the crops and when it would be consumed by humans it would harm then and will decrease their carrying capacity.
