The answer to 'why is my drip edge sagging' is that the drip edge has become loosened or detached from the roof's edge. This can happen due to factors like poor installation, age and wear, or damage from weather or other external forces. Symptoms include visible sagging or gaps along the roof's edge, which can allow water to leak behind the drip edge and damage the roof deck and fascia.
To fix sagging drip edge, you'll first need to inspect the roof edge to determine the cause. If the drip edge is simply loose, you may be able to re-secure it by nailing or screwing it back into place. However, if the drip edge is severely damaged or the roof edge is rotted, you'll likely need to replace the drip edge and potentially repair the underlying roof structure. This is a job best left to a professional roofer, as it requires specialized skills and equipment.