. Seems to me that would mean that the bolts and washers aren't the right profile for the concave sections in the slot.
Not all washers are the same. If you've ever seen the k2 flat washer you know why different they can be not to mention impossible to find. If you currently have any type of foam or dampening material between the board and binding it might be compressing as you ride , jump, or land. If it compresses the whole disc will be a able to move. There is a reason why Burton's screw mounts are as far away from eachother as possible.
The two point mounting system doesn't help because it is designed to flex. As the board flexes , it will push the screw up allowing movement , that is why your screws are still tight when you get to the bottom. You can 1)try to add friction under the disc with padding as some have mentioned, 2) add padding to the outside of the disk to absorb the board flexing 3) try different washer 4) get new bindings.