Because horses are grazing animals, designed to travel long distances to find food and eat whenever they find it available, going for that grass is a perfectly natural behaviour.
As a first step, ensure your horse is getting enough forage (hay or grass) in her diet and that she has eaten before going out to ride. If her tummy is full, she’s less likely to be tempted to snack.
Next, work with her at home so that she consistently responds to your quiet leg and seat aids to go forward. A horse that is moving from her hindquarters can’t lower her head to grab grass as easily. Also, work on your riding position, ensuring you are balanced in your saddle, sitting tall and with your weight evenly over both seat bones. A well balanced rider is not so easily pulled forward out of the saddle.