You're describing "lens creep." This is when a zoom lens, pointed up or down, doesn't hold its focal length position (typically zooming out and making the lens stick out further). It ain't an Apple, but Sir Isaac Newton would still say it's "just gravity at work."
This creepy problem dates back to the early days of zooms: the old push-pull zooms were particularly prone to it, as there's often not much friction on the zoom action. In a few cases, I've seen focus "creep", too, but that's much more rare in these days of internal focusing lenses.
Some lenses have a "lock" position to keep them from extending while you're walking around. Not all lenses have that, though, so what's the solution?
A rubber band.
Yeah, a big, fat rubber band that's wide enough to grab the focus ring. When you don't want the lens to creep, leave part of the band on the ring and move some of it onto the lens barrel itself.
Don't have a big rubber band or prefer something commercial? Yes, there's a product called Lensband that will do the same thing. Or perhaps one of those charity-inspired wrist bands might be the right size for your lens and assuage your conscience at the same time.
