the lift cylinders are "single acting" meaning they only push up, not down, and if oil somehow bypasses the seal in the cylinder rod's piston, then the oil on the top side of the cylinder has to be forced back to the correct side of the seal.
If you lift this unit it's full and complete stroke, and then lower, does it do the same thing on the next stroke?
You may have a 'cushioning valve' in the very bottom of the rod that has gone bad, so you will need to repack the center lift cylinder, complete (not just the gland nut) and replace the cushioning valve and o-ring in the bottom of the rod