Are there leaves on both sides of the tree? Check the trunk for damage to one side of the trunk. Have you sprayed for weeds or something and the spray could have drifted through the air to one side of the tree.  Do you have neighbors who could have done some spraying? Do any of the roots appear to be wrapped around the trunk of the tree? This web site may also be helpful. 
https://www.reddit.com/r/gardening/comments/3o8xe5/why_is_my_apple_tree_only_growing_apples_on_one/