I don't know why this one did.
Thin does not cause cracking, but it certainly can increase the risk that some other factor, that might otherwise be innocuous, could cause a crack.
I would speculate about three possible causes:
1. Overly aggressive sanding. Sanding too fast, or with too much pressure, can cause overheat heating that can trigger a crack.
2. The crack is at the end of the barrel where the bottom finial is pressed into the tube. How do you assemble your pens? Specifically, it is possible that the finial axis was not perfectly aligned with the body axis when you pressed it into the tube? If the pen part being pressed into the tube is not perfectly aligned, it can distort the tube and cause the kind of crack shown here. DAMHIKT When you watch other people assemble pens in videos it looks really simple, but my experience is that it seems to take at least one more hand than I have available to hold everything together while operating the pen press, which means that its entirely too easy for something to not be lined up exactly right.
3. Green wood is notorious for cracking as it dries, shrinking circumferentially and creating cracks that extend radially from the pith outward. While that phenomenon is always a possibility, I would be inclined to not suspect it in this case. Lignum vitae is a tropical wood, and it takes time for tropical woods to progress from green logs to pen blanks; by the time a pen blank is turned, one would normally expect that the wood would have dried to the point where the internal moisture content is similar to the ambient humidity. That said, it is possible that there could be additional shrinkage that is confounded by the constraint that the brass tube can't move and that leads to a crack. How long had you owned this blank before starting the pen? Also, how much time transpired between drilling the blank and gluing in the tube, turning the body, and finishing and assembling the pen?