Oh, I thought you knew these 2 cats since you named them Snowball and Ginger. Just for identification porpoises, huh?
I've got a couple of thoughts.
Snowball and Ginger may have been pissed off at each other, and Dog was an easy target to vent their rage, since fighting with each other would have caused harm.
I have had it happen, with the gentlest cat I had, that I stepped in the wrong place at the wrong time. She was mad at something. I distracted her by walking by, and so I became the target (shrugs, it happens)
Maybe they were male and female and were planning on getting it on (Cue Barry White music) and again, Dog presented as something that was interrupting.
Cats just don't think like dogs. I don't think they were so much cooperating with each other to tag team dog, but when one stepped back, the other felt safe to move in, knowing he wouldn't get swiped by the other cat.
roland and otis, my 2 boys now, play and wrestle with each other, but if I do something with a ball or toy, first one will play with me, then steps back and the other one does. Or one will try to entice the other to play, and the other will ignore him until he damn good and ready to respond.
roland is a people cat, and otis is a cat cat. otis is cordial, likes brushing, playing, but people are not really on his radar screen to seek out. Because to otis I really don't exist often in his world, he's never made any move to act agressively toward me.
roland is a lover boy, but, he is a cat. He does enjoy me bapping with him, pushing him over, rough scratching sometimes, etc.
One time he gave me a pretty good bite, and even then I knew I had no one to blame but myself.
I wasn't watching him as closely as I should, as I was enjoying myself, and missed that subtle "that's enough" sign from him, which needs to be respected.
He turned his head looked at me and I saw his pupils dialate, he's ears went back and he got that crazy look you never want to see. This all happened in like a quarter of a second. I thought "uh oh this is not good" and immediately went into "get his body away from mine" mode. Quick as a snake he latched onto my wrist, but I already had grabbed the back of his neck and tossed him away from me. I felt lucky he didn't come back toward me. Instead he gave a good hiss and slunked off. When he came back later, I swear he was chagrined. I over agitated him, I should have known better. That's the nature of cats. Not that they are evil, not even that they are unpredictable....well, mine haven't been, but yeah, some are I know. They are just not team players, and don't drink the koolaide.
Anyway, I believe those two cats were already over agitated over something the two of them were involved in, and Dog was in the wrong place at the wrong time.