I have had this occur on occasion over the years and it appears that there isn't just one cause. Here are some reasons I have seen:
-The item I sprayed it too soft. I spray my desserts while frozen, but once it thaws, if the center has more give than the coating, it cracks.
-Too much humidity or condensation. I often leave my frozen items at room temp 15 minutes before spraying.
-If the spray coating is too thick, there is less give.