The answer to 'why is my underlayment curling shingles' is that the underlayment is likely not properly installed or has become damaged over time. Curling underlayment can cause the shingles above it to also curl, leading to leaks and other roofing issues. This is a common problem that can be caused by improper installation, exposure to the elements, or age and wear of the underlayment material.
To fix the issue, you'll need to inspect the underlayment and determine the root cause. If it's simply a case of improper installation, you may be able to re-secure the underlayment yourself. However, if the underlayment is significantly damaged, it's best to call in a professional roofing contractor to assess the situation and make the necessary repairs.