As mentioned above, it looks like maybe it should have been cut sooner, but also the knife itself is probably another part of the problem. Kitchen knives with the wedge shape that they have are find for cutting meat and so forth, but not that good for soap. Splitting logs with wedges, you may have noticed, can cause splintering. That's what happens with brittle soap, too. Now, I don't always end up with brittle soap when I make a Castille, but it has happened to me once or twice. Usually when I get brittle soap, it's a combination of ingredients and waiting too long to cut the soap. And when I used to cut with a wedged shape kitchen knife, that just complicated things with brittle soap. In this case, it's not the ingredients, though. So I would say it is the knife and the initial hardness of the soap, which probably needed to be cut hours earlier (when at a sort of mild cheddar cheese consistency/hardness).