We have a little(r) setter here, Grace, who does the same thing. It's just 'her way'. Especially on wet days. It seems her hind paws are much more susceptible to melting in rain or snow than her front feet are, so she alternates holding the hind feet up so as to minimize their time in peril.
But to some extent, she'll do the same thing indoors for no apparent reason.
So it may just be a quirk for your dog, like it is for Grace. But next time into the vet you might want to have them manipulate his spine to make sure he isn't tender along his back. Sometimes a dog with hip or back problems or even just a pulled back muscle will throw their weight forward to relieve pressure on the hind quarters and that will sometimes include the favoring of alternating hind legs...