The answer to 'why is my underlayment sagging' is that the underlayment has become loose or detached from the roof deck. This can happen due to poor installation, water damage, or simply age and wear over time. Symptoms include visible sagging, wrinkles, or bumps in the underlayment material.
Sagging underlayment can lead to leaks, mold, and other roofing issues if not addressed. It's important to inspect the underlayment regularly and repair or replace it as needed. In many cases, this is a job best left to a professional roofing contractor, as they have the expertise and equipment to properly assess and fix the problem.