My Tamron 100-300( Pentax fit) does this slowly but takes several minutes to zoom from 100 to 300 and only when I walk with it in the down position when hanging the strap on my shoulder. Not an issue in normal shooting position. It is annoying and extends the length of the lens enough to making carrying awkward but I cannot think of any easy way to prevent this and still retain the need to alter the zoom.
I think more expensive zooms have a lock which can be engaged and then disengaged for zooming but
it was a "extra" that the cheaper zooms didn't have.