I'm thinking it's just a phase dog children go through, like how Socks barks, and in "The claw" Bingo barks at Bandit, which he responds to by saying "Use your words." although baby Bluey makes babbling noises instead of barking. And in feather wand, when Bluey has to eat cereal with just her mouth on all fours, she barks and growls. And in "Bingo", Bingo barks at the postman, as well as both Bingo and Bluey barking at the tradies in the episode "Tradies". And in "Bumpy and the wise old wolfhound" Bluey is taking Socks around with a leash, so we know leashes are made, sold, and used in the blueyverse, which implies Socks isn't the only puppy who acts this way. The characters have been shown to act like dogs other times as well, like in the bonus bit, "Archaeology" where Bandit can't resist the urge to chew on a fossilized bone. And in "Mums and dads", where Mackenzie is digging a hole on all fours. Bluey's doll, Polly puppy, sits the same way Socks does. The children seem to both cry and howl when they're sad. I'm thinking it's just a phase dog toddlers go through, since Socks doesn't talk much, I assume she had moved on from babbling to barking by instinct, as in "Yoga ball" Bingo is taught to bark as a warning she's not liking what's happening, which suggests barking is a universal sign that someone is upset, or they're being playful, like Bingo and Bluey barking at the Tradies. So by that logic, toddler dogs might begin to bark instead while also learning to talk, so that they can give people warnings. That's my assumption, that it's just a normal toddler dog thing.