This is actually a natural behavior! European herring gulls like this one – and many other bird species – usually do this sort of “dance” on soil to hunt worms. The pattering imitates the sound and vibrations of raindrops, bringing the worms up to the surface to avoid drowning in their burrows, only for them to become easy snacks for the bird.

This particular gull has obviously learned that it can get far more food if it does this behavior in front of humans instead.