Maybe something happened while they were in Japan, who knows....
Actually, something that will be explained in the future, cause it makes no sense to start the series like this.
I didn't read the novels at all, but according to what I know, it really is strange...maybe something about Kazuya's Family or about Victorique herself
At the beginning Gosick never makes sense until we reach some explanation part, so, we'll know in the future.