Always just a little baffled. Since he comes after Grandma twice, it seems like he holds a grudge, and going after her family implies a... hereditary bend.
However, half his victims are their in-laws, which implies that he goes after everyone, they're collateral damage, victims of circumstance, based on one marriage.
Or maybe it doesn't even matter and I'm over-thinking.
Actually just watched it again (threw it in the Christmas movie rotation a few years ago, I really enjoy it). I started wondering the same thing myself after I realized the delivery man was killed. I assumed Krampus coming back for Omi's family was random (her grandson randomly brought Krampus down on them).
But as for rules, I don't think he has any. Aside from maybe only being able to hunt on Christmas Eve?
Ghost Riders in the Sky
They gave this Krampus elements of Herne/Cernunnos the Hunter who is said to hold a ghostly wild hunt every winter. He takes everyone in his path and they are doomed to hunt with him. He wears horns and carries chains.
In this film, it could be the goblins and gingerbread men were past victims.
Krampus did not go after the Grandmother specifically, it was just happenstance that Max attracted Krampus.