"Dogs, like everyone, don't like the sun in their eyes." would be enough of an explanation for me. Predators even don't like the sun in their eyes, that's why they attack with sun behind them, if they can. Wouldn't make much sense for the dog to turn it's back in the most likely direction a predator is going to attack.
Some people like heated toilet seats. Maybe most dogs like some warm sunhine on their behinds while pooping.
Or maybe they want to face upwind for other reasons.
While I have no doubt that predators don't like having the sun in their eyes, they would obviously still prefer to attack with the sun behind them even if they didn't care. It's very advantageous, because the prey doesn't like having the sun in its eyes.
Attacking with the sun behind you is to prey with eyes what attacking from downwind is to prey with noses.
Cats, on the other hand, are carnivores.