Phone cases can turn yellow for a variety of reasons, including:

1. Exposure to sunlight: Clear phone cases are often made of polycarbonate, which can become discolored or yellowed when exposed to sunlight or other sources of UV radiation.

2. Age and wear: Over time, all plastics can break down and become brittle, which can cause discoloration and yellowing.

3. Environmental factors: Exposure to heat, humidity, and certain chemicals can cause clear phone cases to yellow over time.

4. Smoking: Smoking can also contribute to yellowing of phone cases, as the smoke and tar can leave behind a residue that can discolor plastics.

To prevent your phone case from turning yellow, you can take a few steps. First, avoid exposing your phone to direct sunlight or other sources of UV radiation for prolonged periods of time. You can also store your phone in a cool, dry place when not in use to help prevent discoloration. Finally, avoid smoking near your phone or phone case to prevent the buildup of residue that can cause yellowing.