the mast should have a safety valve in the base of the rod of the freelift cylinder. This safety valve may be bad or may have come apart. I have seen this happen alot in many brands and models of trucks. I would take out the freelift rod/piston and check this.
There is also a velocity fuse or safety flow control valve in the supply line either at the base of one of the primary cylinders in a special adapter fitting or a valve in a manifold block mounted on the rear side of the mast where the main lines go to the cylinders, this controls decent speed in case a line ruptures to keep it from dropping too fast, this is not a common scenario but is possible and should be considered looking at if the safety valve is not found to be an issue.