The answer to 'why is my drip edge sagging' is that the drip edge is likely not properly secured or has become loose over time. Drip edges are metal flashing installed along the edge of a roof to direct water away from the home's siding and foundation. If the drip edge is not securely fastened, it can start to sag or pull away from the roof.
Common causes of a sagging drip edge include poor initial installation, nails or screws working themselves loose, or damage from weather and age. Symptoms may include visible gaps between the drip edge and roof, water stains on the siding, or pooling water at the roof's edge. To fix a sagging drip edge, you'll need to re-secure the flashing, either by re-nailing or screwing it back into place.