I’m tempted to say sandworms, but it’s not the same Dune.
In White Sands National Monument, the dunes will creep over a tree. The tree eventually decays, leaving a trunk-shaped hole in the sand. Hasn’t happened recently, but in the past, people walking over such a cavity have fallen in, and usually the cavity collapses.
So maybe there are old trees or telephone poles under the dune.
They should put out thumpers, just in case.
Used to be called subsidence. Years ago when coal mining lots of trees were buried, as they rotted away houses built on top of them sank into the ground. Also happened from the collapse of old underground mine workings, coal and rock.
Or it could be buried treasure and the traps left behind, think Oak Island, are finally rotting away. Or could be an X-File.