Why it happens
Pastry gets tough when too much gluten develops. Gluten forms when flour is mixed with water and then worked, so kneading, over-mixing or re-rolling the dough all make it stronger and chewier. Adding too much water gives the gluten more to work with. Fat keeps pastry short and tender by coating the flour, so too little fat, or butter that has gone warm and soft, also leads to tougher pastry.