Technically, they are not dinosaurs because they are not descended from the common ancestor of all dinosaurs, which had limbs so configured. Some "allied" line

Technically, they are not dinosaurs because they are not descended from the common ancestor of all dinosaurs, which had limbs so configured. Some "allied" lineages had them, too, omitted from Dinosauria proper for technical reasons. Some isolates developed it independently, mammals notably among them. Probably some dinosaurs developed a sprawling gait "secondarily". Biology is a messy business.