Answer:
Selective breeding of normal sheep with too much wool sheep.
Step-by-step explanation:
Selective breeding can lead to some sheep overheating from having too much wool if the sheep of having more wool is crossed with normal wool so the offspring is produced with huge amount of wool on their body that keep its body warm in winter season and overheat in summer season. So the sheep shed their hairs in the summer season in order to reduce overheating of the body.