Some cultivars are more susceptible to cracking and splitting than others.  It is also impossible to say what the nature of the soil is under the trees even though your trees are only 8 feet apart.  Rocks and springs can be anywhere.  In the future, if there is not an inch of rain in a week you could consider irrigating your trees to keep the moisture level even.  The splitting is almost certainly due to uneven moisture in the tree and irrigation might help.  vw