Cracks in tree bark can come from previouse frost damage or drought damage. The following is a good article on the subject.
Many arborists and landscapers often suggest to their inquiring clients that frost-cracks are the reason for the bark separation within the trunks of trees. Often times, however, the actual cause may be from drought stress. Drought-cracks occur most commonly when trees are first planted and a year or so after transplant. The trunks will crack if trees are allowed to become too dry. Some common trees more susceptible to this condition include maple, honeylocust, crabapple, mountain ash, and London-plane. One of the easiest ways to distinguish between a drought or frost crack is by the way a wound closes. Drought cracks typically represent a one-time event and will generally close or seal almost completely and never re-open again. On the other hand, frost cracks are more likely to continue to open and close over subsequent years. Usually less freezing and thawing stress is required to re-open the crack in the future and the development of a conspicuous callus ridge often occurs over time. Sunscald wounds occurring on the south or southwest sides of tree trunks can also create cracks (i.e., frost or drought cracks can occur on trunk sides facing any direction). Sunscald wounds generally never close or seal over and may often increase in size as the tree grows. Although there is rarely any need for immediate concern when managing trees with these types of trunk cracks, the open wounds can increase the potential exposure to wood decay fungi.
Information from Steven K. Rettke, Ornamental IPM Program Associate, Rutgers University.