This is a very imprecise question. Is this specific for a strain of animals? Is it genetic and heritable? Does it cause a phenotype that is deterimental to fitness? Lot of genes are expresed in cells that may be superficial or unnecessary, but if they cause no harm then the cell has no reason to suppress expression. When you say earlier, do you mean in embryonic development? If so then the cells are likley to see a signal prematurely. They may have increased sensitivity to a peptide or morphogen, perhaps due to a variant in the receptor. Or, they may be missing a repressor that acts to inhibit expression early but is then switched off later. There are many models for how this could happen.