How to fix rocking stone slab stair?
One approach is to pry up one of the tottering edges up slightly and inject exterior construction adhesive (some are rated for stone/masonry) a few inches in and staying well away from the outside edges. Then lower the stone and let it dry undisturbed.

When dry, this creates an adhesive wedge. Its sticking power is less important than its shape.

Obviously, if rearrangement or removal of the stone is someday needed, this makes it a bit more difficult, but not impossible.

However, if you think the problem is progressing, you need to address the underlying erosion problem before taking this step.