The answer to 'why is my underlayment leaking' is that the underlayment, which is the protective layer installed beneath the shingles, may have been damaged or improperly installed. Common causes include improper installation, age and wear, or physical damage from things like nails, screws, or heavy foot traffic.
Signs of an underlayment leak include water stains on the ceiling, damp spots on the roof, or mold growth. To fix the issue, you may need to repair or replace the underlayment. This is generally a job for a professional roofer, as it requires specialized skills and tools to safely access and work on the roof.