There is no evidence that the pits were discrete depressions in the topography that filled through time. Rather, they appear to have been highly localized areas of liquefaction caused by large-dinosaur (possibly sauropod) trampling of saturated sediments.
Does this mean the vibrations from trampling sauropods liquefied nearby quicksand...? That would be awesome.
In any case, the paper shows that size does matter. =8-)