The wall has weep holes, right?
Use a post hole digger to sink a vertical hole in back of the wall at least 6' deeper than the bottom of the wall and
fasten the wall to an upright post in the hole with lag bolts and fender washers. This should resist the tipping torque at the surface of the ground and will also prevent the wall from sliding out.
Otherwise you could put in long horizontal sleepers just under the surface and fasten them to the top layer of timber and drive vertical posts/pipes through them to allow them to resist the horizontal force causing the leaning.
This will not prevent the base of the wall from sliding out or the wall from bulging.