People here really trying to claim those maids and other servants got out safely either have no idea how fast an avalanche moves or are just being willfully ignorant of their death.
On page 27 we get a nice clear view of the castle from the hill Karina is standing on and we can see the only sides with slopes the avalanche could come from are the right, or behind it. On page 30 we can see it crashing from behind the castle and moving very clearly in the same direction as the caravan leaving the castle and literally right on them screaming for help, then on page 31 from the same hill we could see the full castle and surroundings the entire area is now buried in snow and debris that is still flowing down the path the caravan was leaving on.
They didn't outrun it, they didn't just step to the side of it, those people are 100% buried and gone, and any writing to say otherwise means either the author fucked up the art(since they are also the artist) or they are ass-pulling them from the grave.
Now on the topic of who caused it, we simply don't know at this time. It could've been Karina asking for it, it could've been the sister thinking she was still in the castle, hell it even could've been natural, we don't know.