Boy, an awful lot can be going on in a situation like that, and can be missed by all the owners because it happens so fast. You didn't say whether Marley is neutered, how old she is, and the size difference between her and Snickers. If she hasn't yet been neutered, it could be sexually driven. Even if she HAS been neutered, bitches aren't called that for nothing.
If she is adolescent, she may be feeling her oats, and took it out on Snicks.
The GOOD thing is that dog "fights" that end with no blood were not meant as real "fights". If she didn't draw blood, she was giving Snicks a "dressing down" and didn't mean to hurt him. Snicks did the right thing by going belly up, to show that he had no interest in fighting back. Did she then separate from him, or did a person have to pull her off? If they separated on their own, even if Snicks then ran for cover, Marley got her message across, and Snicks is unlikely to do whatever bothered her so quickly again.
It MAY be that Marley is resource guarding either her person, or more likely, the treats he was carrying. This is often a bone of contention between dogs, and it could be that you could maintain peace by making play group a "no treat" period for everyone to avoid resource guarding.
I think that you are right, Marley probably had NO idea why she was disciplined. It was the right thing to do to get her back on leash if you folks didn't know why/what happened, but if the discipline came AFTER leashing her (or even catching her), as is most likely the case, all the owner REALLY reinforced is that it's better not to be caught or leashed. Probably NOT the message they were trying to get across!
If it were me, and Snicks and this dog are close to the same size, I'd try it again, without any food present. If the other dog is much larger, it is more problematic, because Marley could hurt Snicks more than she meant to, just because of the size difference. But just making a lot of noise and one dog going belly up for the other is the equivalent of two siblings squabbling... not that big a deal. Many people with two or more Havs living in the same home see the same type of behavior from time to time within the home.
If the problem continues, even with no food present, and it's important to everyone to be able to continue the group, see if everyone would chip in to have a good positive based behavior specialist come and watch the group dynamics to see if they can figure out what is going on. It wouldn't be expensive for anyone if you all chipped in.