It could be cooler temperatures effecting the shape of the leaves as they mature. The new leaves growing out now look normal and when they open, if they remain normal, it probably isn't due to an insect attack or micro-nutrient deficiency. How often do you fertilize it and what do you use? It looks like you have mulched it in the past, make sure that you keep about a 3 inch depth of mulch around the tree. A good fertilizer with micro-nutrients and mulching will ensure the tree will be as vigorous as possible and suffers less from other stresses. If the tree starts to do this again next winter, it may be just the cooler temperatures alone that cause the curling of the leaves.