Why does my focus plane look diagonal in sports photos?

Your lens is demonstrating tilt. In addition to the bottom to top negative tilt (the bottom is in focus closer that the top of the frame) there also appears to be a little positive tilt left to right (the left is in focus slightly closer than the right). Sometimes the lens gets out of alignment due to a hard blow or fall. If that is the case the lense will demonstrate the same characteristic, to one degree or another depending on focal length, focus distance, and selected aperture, in every shot. Sometimes the image stabilization components (VR in the Nikon nomenclature) can create tilt that is more variable in terms of how often and in what direction it occurs.

Do note that if you are shooting at an angle from an elevated position, which doesn't seem to be the case in your examples, a lens with a proper flat plane of focus will show the lower part of the frame in focus in front of subjects standing straight.

The lens needs to be adjusted by a service center qualified to do so, and if the lens is in warranty by one that is authorized to do such repairs by your lens' manufacturer. How effective such a repair will be can vary greatly from one instance to the next. Roger Cicala, the founder and lens guru at lensrentals.com, has addressed the frustration of having a misaligned lens returned to a customer and labeled "within specification" at this entry to his blog.