Sometimes swans will pull aquatic vegetation in the manner you describe to eat. However, if this is land based reeds/grasses/vegetation, then the swan is just clearing the area. Usually, this behavior occurs frequently during nesting season.

Swans are a grazing bird, so they will tug at vegetation to eat or clear areas, especially around flower beds, gardens, hedges and grasses. We have seen them clean out and strip white pampus grass just to sit in the area for shade. We hope this information is of benefit. The Regal Swan