Not gonna lie, I think it was Sans.

Firstly, they don't recognize who attacked them. Yes, it was dark, but they would probably be able to see a silhouette at least, otherwise how would they know anything was there in the first place? And yet they don't know who did it. Sans and Paps are new to town, so the dogs might not be familiar with them.

Secondly, we know how Sans gets when he's pissed off. In UT it was only after you killed literally everyone, but maybe the Sans in Deltarune's universe is a bit more quick to act (or just trigger happy).

And finally, the dogs say they were sneaking into a big house. Well, out of the houses we've seen so far, the skeleton residence is the biggest. It's possible this was supposed to be the Holiday's home, but why say "big house" and not "mansion" then?

This concludes my TED talk on why Sans attacked a bunch of dogs, maybe.