Bill Tindall, E.Tn.
Because of the way tree cells are assembled the circumference of the log will shrink about twice as much as the radius as it dries and the cells collapse. As a result "pie shaped" splits will develop around the circumference to relieve the stress of the circumference wanting to be smaller than the radius will allow.
If you split the log before drying the half log may just curl to relieve stress and not split at all.
The rate of moisture loss out the ends is about 10 times the rate through the circumference. This difference is why ends split unless drying out the ends is slowed.