It happened twice.
Some bipedal theropods and bipedal theropod-like archosaurs evolved to become quadrupedal crocs and quadrupedal croc-like theropods.
Most famously and most recently,
Spinosaurus (Fig. 1), long suspected as being a fish-eater, was reconstructed with shorter than expected hind limbs, thus forsaking any ability to walk on its hind limbs alone. Some workers think this chimaera reconstruction is bogus. Others are more accepting. Spinosaurus was derived from bipedal theropods like Suchomimus and Sinocalliopteryx. Deinocheirus was giant frill back bipedal theropod related to Spinosaurus.
Basal bipedal crocs evolved to become extant quadrupedal crocs
Basal crocs were bipeds (Fig. 2), only later shortening the hind limbs to become quadrupedal, like the theropod dinosaur, Spinosaurus (Fig. 1).
It’s of mild interest
to note that the transitional croc, Sphenosuchus, had taller dorsal spines than either more primitive or more derived taxa (Fig. 2).
Contra this pattern,
the finback Arizonasaurus is a likely biped (based on its deep pelvis) derived from quadrupedal, shallow-pelvis, basal rauisuchians, like Vjushkovia. Even so, the closest relatives of Arizonasaurus include croc-like Yarasuchus and Qianosuchus both of whom have semi-tall spines.
A relative of Arizonasaurus
by analogy, not homology, is Qianosuchus (Fig. 5). It shares many traits with Spinosaurus, sans the frill.
Ibrahim N et al. 2014. Semiaquatic adaptations in a giant predatory dinosaur. Science 345 (6204): 1613–6.