Either your pastry dough got too warm while handling and melted butter leaked out in the oven, or your oven temperature wasn’t high enough. Puff pastry needs to be cold and baked in a hot oven to rise properly. Next time, if you notice your pastry dough gets too warm after rolling it out, pop it in the fridge for a few minutes to firm it up again before adding the toppings. And make sure you preheat your oven to 400 degrees F, no lower.