The first respondent is right about what the gulls are doing - they are causing vibrations to irritate the worms to the surface (it is possible that they respond to the vibrations because they are similar to those caused by rain, and the worms head upward to avoid drowning in their burrows).
This is nothing new, however - some species of gulls have likely been doing this for millenia. I think the issue here is that the name "seagull" is a misnomer - many gulls have nothing whatever to do with the sea. That's why they are properly known simply as gulls. There are species of gulls that nest inland in the north and forage on croplands without ever seeing the ocean.
Gulls are very smart and very adaptable birds, and that's why most species are doing very well. To the best of my knowledge, the only gull species currently threatened are those living in the far north such as the ivory gull. As our climate warms and the ice retreats, the animal kills that these gulls scavenge from large predators are growing harder and harder to find as the predators themselves are diminishing in number. Also, the traditional nesting sites of these birds are no longer as protected as they had been in the past.