I'm slightly in the same boat. my girl for over a year now (she's almost two) had been really great when i take her to the stores or other pet friendly places as I also like to try and keep her socialized as much as possible, especially now that our other two dogs have passed away in the last couple of years since we had her.
Lately, she's randomly started to growl and bark at people, but then she'll be fine or seem fine with everyone else O.o I'm not sure what it is about those specific folks that triggered it. lol it does get a little embarrassing when it's at a pet friendly store like the crafts stores we have here, but I guess I should've expected it to happen at some point. She never has done it at the pet store (knock on wood) but she's been going there often since she was a baby. Since our old girl Chloe past away, she's become more of a guard/top dog. I guess she just feel much more protective now. She'll be in my room and even sometime bark at family if they suddenly pass by or suddenly open the door at night. In those cases I thinks she's just more easily startled.
I'm thinking in the cases of her barking at random people, it's probably because she's not familiar with the place and doesn't feel as secure. I've recently started taking her to new places when this started to happen. I've also put her in the cart with just her carrier mat, instead of putting her in her carrier, into the cart at one place that she started to bark at folks. It makes her feel more out in the open. She loves it but I think it also made her more aware of her surroundings. She's a little bit more better if I keep her in her carrier as the most she's done was low growl (so far).
I do eventually plan to get her some professional obedience training, just need to work out how to get that going with work and stuff ^^; Or if all else fails I can just try and look up training tutorials
So yea, I think being in a new and especially big place is one thing that's triggering your baby's response. It may help to try those calming pheromone collars or drops, but training is definitely a must.