Cutter threw pebbles and broke 2 rear window glasses today

Kuhn GMD4050TL threw pebbles and broke 2 rear windows today. Field is sandy and has pea gravel in soil. There are a few gopher mounds and both times glass broke when mound was hit. Don't know why curtain is not stopping pebbles. Any suggestions as to how to prevent this? Cutting bermuda. Thanks

Cutter threw pebbles and broke 2 rear window glasses today