There could be a few reasons why this might happen. If you haven’t cooled them long enough they could tear as you cut them. But that brings us to the most common reason, you are using a dull knife! You can use a serrated knife and “saw cut” the biscotti with good results. If you have a normal knife that is sharp you can cut them just as well, also using a saw cut. It is possible that you added too many things into your biscotti as well, so be careful if adding extra ingredients. This recipe has about as many almonds in it as possible without causing the loaf to lose its cohesiveness.