I get Ground Type Pokes live underground/likes to dig (Diglett, Sandshrew, Drilbur, Trapinch, Diggersby, etc) or are made of ground/sand/dirt (Sandygast, Baltoy)
But what's the deal with Garchomp, Seismitoad, Gastrodon, Whiscash, Swampert, Nodoking, Nidoqueen, Gliscor, Marowak, etc.
A shit ton of them are extremely random.