It could be anything from a smell he doesn’t like (like it’s not being cleaned properly in the groomer’s trailer or maybe he’s getting the hose in an area that might stimulate his poopmaker.
Also, that groomer should have assured you of how common it is. At my daughter’s dog day care they go all day long all over the place, but especially with the groomer.
Update: I called the grooming company owner, “Lulu,” to schedule an appointment. Now that spring is here, Buddy needs to shed his winter coat (I could probably knit a whole new dog from the extra fur). BTW, this mobile groomer is the top-rated dog groomer on Yelp in my city. I’ve been using them for a couple of years.
I told her about how Buddy pooped twice with Roberto, and this was news to Lulu. And that Buddy had never pooped with the previous groomer (who had to be fired for being a chronically late flake.) I floated the idea of anxiety possibly being induced by a man speaking Spanish because of Buddy’s history, and she got it right away. She said she has several customers whose dogs (especially rescues) are afraid of men, and she sends female groomers to those households.
I haven’t met Lulu, but she has a heavy accent that might be Hispanic… but for some reason, I think of her as Asian. I dunno. So she’s sending me a plain-white-bread “American” (her word) woman on Friday, and we’ll see how the Bud-Man does with her.
[del]Repoop[/del] Report to follow.
I’m glad that Lulu understood and was happy to make the switch for Buddy’s sake.
Good news! The Budster did great with the new female groomer Natalie (who BTW was precisely on time to my house-- a marvel in itself!). He even let her brush his teeth! He is such a good doggie and now he’s the cutest boy-dog at my house.
Life is good.
Yay! 
Harmony is once again restored to The Dog Universe. 
Take your dog for a walk before the groomer get to your house and try to watch how your react when he see the groomer. I took my dog to a groomer and my dog tried to hold onto my legs with begging not to leave him there , I stopped using that groomer. Your dog could also be smelling another dog’s poop in the shower and think it’s OK to go there. Dogs do at the vet’s office all the time , especially male dogs. They pee right where another other dog peed my male dog did a few times and I had him pee before going inside the vet office.