Pastry dough can shrink because the dough is overworked, stretched too much, or not rested sufficiently before baking.

Think of the gluten strands in the dough like rubber bands: if you stretch the dough while rolling it out or don't let it rest long enough in the fridge, those bands stay tight. As soon as the heat hits them in the oven, they snap back to their original size, causing the pastry to pull away from the edges and lose its shape.