Two possibilities: the butter wasn't kept cold or the dough was overworked. Cold butter does not fully incorporate into the dough, which means there are small pockets of cold butter throughout your pie crust. When the crust goes into the oven, the water within the butter evaporates, leaving a small air pocket, which results in a flaky crust. But if the butter is warm or the dough is overworked, the butter--in effect--gets "smeared" into the dough and you lose those small pockets of butter necessary for a flaky crust.