There may be a couple of reasons. Firstly, make sure to chill the pastry before rolling it out and aim to keep it cool. Try freezing your stainless steel rolling pin before rolling the pastry - it really does wonders at keeping the pastry cool. What is more, you may have used the pie dish that was simply too big. Ideally, you want the dish almost full of filling. The scattered halloumi cubes on top will provide support for pastry that will rest on top and won't allow it to touch the cream, keeping the pie top relatively dry and preventing it from sinking.