How can carrying capacity be reduced?
For example, if a grassland is becoming hotter and hotter, drying up, and turning into a more desert-like environment, then the number of individuals living in that grassland is going to decline as suitable habitat decreases.
Carrying capacity could also be reduced if each individual within the species consumed less from the environment. Think about humans: if every human needs a four car garage and a large house, the planet can sustain fewer humans than if each human lived in a studio apartment and traveled using a bicycle.
Check out this link for more information on populations and carrying capacity.