While I cannot know for certain without being able to examine them closely, I would suspect the paper. Paper is not volumetrically stable, as anyone who has taped the four corners of a sheet to a surface knows. Over time, the paper expands and contracts, primariyl with changes in humidity.
Added to that, the decal film (the clear coat that carries the ink) is a polymer. All polymers shrink over time, those that are intended to be flexible shrink more than those that are rigid. Paper expansion and contraction plus coating shrinkage yields shattered decals.