The answer to 'why is my underlayment hail damage' is that hail impact can cause significant damage to your roof's underlayment. The underlayment is the water-resistant barrier installed beneath the shingles, and it's susceptible to punctures, tears, and other types of hail-related damage.
Hail can dent, crack, or even completely penetrate the underlayment, compromising its ability to protect your roof from water intrusion. This can lead to leaks, water damage, and potential structural issues if left unrepaired. The severity of the damage will depend on factors like the size and density of the hail, as well as the age and condition of the underlayment.