The answer to 'why is my asphalt shingles sagging' is that the shingles are likely not properly secured to the roof deck, causing them to sag over time. This can be due to poor installation, age and weathering, or damage to the roof structure underneath.
Some common causes of sagging asphalt shingles include nails that have worked their way loose, rotting or damaged roof decking, and excessive weight from accumulated debris or water. Sagging shingles can allow water to pool and lead to leaks, so it's important to address the issue promptly. In many cases, the solution may require professional roof repair or replacement.