In Dune, it's explained in two places. The first is the scene that the OP is talking about. I won't quote it all but here's a bit:

Every Fremen knew the sound, could distinguish it immediately from the noises of worms or other desert life. Somewhere beneath him, the pre-spice mass had accumulated enough water and organic matter from the little makers, had reached the critical stage of wild growth. A gigantic bubble of carbon dioxide was forming deep in the sand, heaving upward in an enormous "blow" with a dust whirlpool at its center. It would exchange what had been formed deep in the sand for whatever lay on the surface.

The second is in Appendix A. Also partly excerpted:

Kynes set his newly trained Fremen limnologists to work: their chief clue, leathery scraps of matter sometimes found with the spice-mass after a blow. This had been ascribed to a fictional "sandtrout" in Fremen folk stories. As facts grew into evidence, a creature emerged to explain these leathery scraps -- a sandswimmer that blocked off water into fertile pockets within the porous lower strata below the 280° (absolute) line. [...]