I am an engineer so if this is technical don't blame me, write to my school!
The mechanism that zooms most of our lenses is a helicoid, a screw thread. *Technically a helicoid is one of the minimal surfaces of mathematics but (fortunately) that strict definition means nothing to us.* For our purposes the difference between a helicoid and a screw thread is the pitch angle, the number of threads per mm. A helicoid is made to impart linear motion and generally has a steep pitch angle while a screw is made for clamping and has a very shallow pitch angle. This works well as the helicoid gives full zoom motion for only a half turn or so, where a typical screw thread would take many turns.
The problem is backdrive, or how easy it is for us (or gravity) to push the screw against it's threads. The steeper the pitch angle the easier it is to backdrive, to the point where the lens extends by itself due to gravity. To prevent backdriving the lens manufacturer can provide a precision fit of the screw and "nut" (and make them from good materials) but that is expensive and still wears over time. The more recent magic weapon is ultra high viscosity grease that almost sticks the helicoid together. This provides a very nice helicoid feel and operation while allowing less expensive materials, looser tolerances, and it is cheap!
http://www.japan-hobby-tool.com/english/oil/index.html
The grease migrates, loses viscosity, and hardens through use and time, and the mechanism wears, all conspiring to let our lenses creep! The fix would be to replace the helicoid and/or the grease but that requires disassembly of the lens and even though the parts are probably not expensive the labor could be enough to make it worthwhile to just buy a new lens. That wasnt what you wanted to hear, was it?
The good news is the looseness does not really affect image quality (unless the lens is so loose it wobbles), it is just a nuisance. The wristband is looking pretty good by now!
