If you drive in a dusty area, that might be the cause. The wind displaced from the front of the car as you drive at speed rushes around the corner of the windshield in that area. My general recommendations to preserve a car’s finish with a minimum of bother
- park out of direct sun when possible
- rinse it off with a garden hose once or twice a week.
- wash it with warm soapy water and a sponge only when necessary, as soap and warm water removes some of the wax layer
- wax with an easy on easy off type of wax twice a year