He could just be being greyhound contrary but equally something could have spooked him recently - just because you didn’t notice it, it doesn’t mean it didn’t happen! Are you able to walk him a slightly different way- turning left out of your gate if you normally turn right, for example? Or take him out through the back door instead of the front? A little change might help to break this habit.
Also, how old is he? Just wondering if his eyesight might be starting to fail him, and he might be seeing shadows that are freaking him out a bit