Seasoning checking is a natural part of wood shrinkage that occurs as the moisture in the wood cells dry. Wood shrinks as the moisture decrease and swells when the moisture increases.
Generally we run into seasoning checking being a problem in timbers, but it can affect board stock as well. When we sell timbers we like to recommend that our clients purchase Free Of Heart Center (FOHC) material because it removes the Pith / Core of the tree from the timbers. Having the Pith of the tree in your timbers will both increase your chancing of seasoning checking and twisting.
Drying of the material will also have a huge effect on the seasoning checking. We try to get most clients to purchase timbers Kiln Dried so that they are given a nice even drying to begin their seasoning process. For West Coast timbers, the standard for Kiln Dried material is 19% 1″ into each face of the timber. This will mean many timbers will not be “core dry” and will still have the potential for checking. This makes it important to keep your timbers out of the sun, wind and rain and let them continue to dry in a neutral environment after delivered to the job-site until installation.
An interesting approach to dealing with the potential of seasoning checking is proposed by Gene Wengert, who informs cautious builders that they can “Take a Skil-Saw and make a saw kerf about 2″ deep, full length. This kerf will effectively relieve the stress fairly well on the three remaining faces, so with controlled drying, checking will be minimal or nonexistent.”
In the end lumber is a natural product and some checking is bound to occur. Homeowners need to understand this risk, but they can also be proactive in doing what they can to minimize this risk as well.
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Timberline Blog 
Once a check begins or is noticed, filling ut with clear silicon caulk will prevent it from getting worse.