The answer to 'why is my flashing sagging' is that the flashing has become loose or detached from the roof, causing it to sag or pull away. This is a common issue that can be caused by a few different factors, including improper installation, wear and tear over time, or damage from weather or other external forces.
Sagging flashing can allow water to seep in and cause damage to the roof, walls, and interior of your home. It's important to address the issue promptly to prevent further problems. In some cases, you may be able to re-secure the flashing yourself, but if the damage is extensive or the flashing is severely compromised, it's best to call in a professional roofing contractor to assess the situation and make the necessary repairs.