As Mal said, when you press the nib into the brass tube, the tube wants to expand very slightly. This imposes tension across the grain of the wood surrounding the tube, and can lead to cracks.
In theory,this shouldn't be a problem - the diameter of the male portion of the nib should be sized to fit snugly into the ID of the tube without causing excessive expansion of the brass.
In my experience, there are three factors that can cause cracks to appear. One is tapering the wood too much at the nib end of the barrel. Obviously, you must turn it down to match the OD of the nib, but if you keep the body of the barrel a bit thicker, and make the taper fairly short, the wood will be stronger. And by the way, the degree to which this matters probably varies with the species/age/dryness of the timber you are using. Burls, and timbers that have a natural tendency to splinter (such as birch) are more problematic.
Another factor is getting glue inside the brass tube. That reduces the ID which means that the tube will need to expand just a tiny bit more to receive the male portion of the nib, which increases the tension across the grain of the wood. Using a plug to seal up the end of the tube before gluing it into the blank is one solution (dental wax, plasticine modeling clay, or the old standby - a slice of raw potato - make good plugs).
Finally, there is another issue that I think is even more likely. If the nib isn't aligned exactly on the axis of the tube, pressing the nib into the tube will cause the end of the tube to distort and that will increase the likelihood of a crack. DAMHIKT