In looking at everything in your pictures it looks like your backing is loaded unevenly, so I would start with reloading it and seeing where that gets you. I too load my quiltbacks with the seams vertical rather than horizontal. Here is a great tutorial....
https://youtu.be/sTwwkAI7YM0. If that doesn't work, then try reloading the back with the seams horizontal and if all else fails then the backing may need to be redone. Good luck, please let us know the solution you found!!!