The answer to 'why is my architectural shingles sagging' is that it is likely due to improper installation or underlying structural issues. Architectural shingles, also known as laminated or dimensional shingles, are made up of multiple layers of asphalt and can become misshapen or sag over time if they were not installed correctly. This can happen if the shingles were not properly nailed down, the roof deck was not sturdy enough, or there was inadequate ventilation in the attic.
Sagging shingles can also be a sign of more serious issues like water damage, rot, or even structural problems with the roof framing. If the sagging is significant or accompanied by other visible signs of damage, it's best to have a professional roofing contractor inspect the roof to determine the root cause and recommend the appropriate repairs.