The cause lies in the pairing of materials. On the StarWalker of the Metal Rubber line the upper section carries a metal thread that engages a threaded plastic bore in the lower section. Every time the pen is opened and closed, the harder metal takes a little material off the softer plastic, so fine shavings collect in the metal thread and the joint gradually develops play. This affects the series as a whole rather than being an isolated faulty example, which is why collectors take a dim view of the design on a pen that cost around 285 euros at the time (as of 2006).
It can be forestalled by lubricating the thread very thinly indeed. A trace of petroleum jelly or another resin-free and acid-free grease reduces the friction markedly and checks further wear; cooking oils are unsuitable, as they gum up and turn sticky over time. Apply it sparingly and wipe away any excess, so that it cannot reach the refill or the rubber sleeve. It also helps to make sure the threads engage cleanly when screwing the pen together and not to open the upper section more often than necessary. A plastic thread that is already worn out cannot be reworked; the only remedy then is to have the part replaced through customer service.