I, too, have often wondered about this phenomenon. While I don't
have an exact scientific answer, I do have a guess, and I'm sure
everybody else has their own, as well. A chain of any design with
(or without) a pendant that has a simple jump ring bail probably
does eventually work itself around over time. But my guess is that a
pendant with a more complex bail, such as rabbit ear, tapered,
slide, enhancer, etc...you get the idea...is usually not quite
symmetrical. And, even if it is, very few pendants are ever
perfectly balanced, weight distribution-wise. No matter how close,
my opinion (please note the word opinion) is that any pendant would
tend, no matter how slightly, to be somewhat heavier to one side. As
the wearer moves about in their day-to-day routine, one side of the
pendant tends to "grab" or "ride up" the chain.
This "guess" of mine can also allow the consideration of the texture
of the chain, itself. Would a chain with more texture (something
with more edges to allow the bail to "grab" better) allow the
pendant to "ride up" the chain faster? Or would that same texture
tend to grip the wearer's skin more so that the chain wouldn't slide
around the neck as easily?
Over the years, I've noticed that some chain/pendant combinations
that I've worn have exhibited this phenomenon more than others and,
until now, I had just accepted the fact that "it happens".
To add to the confusion, I have a favorite pendant that I've worn
daily for many years. It's a very large opal, I set in 14K. I've
worn it on a 1mm twisted rope chain that was a gift from my mother
and, after wearing it for about 2 hours, the catch never failed to
turn around until it got to the bail, where it could go no further.
Well, the chain came with what I suspect was meant to be a 7" ladies
bracelet, as it is merely a 7" version of the same chain. Mom
presented it as an "extender" for the chain. The point is, when I
wear the chain with the extra 7" segment, it takes the better part
of a day for it to perform this frustrating antic.
Anyway, to make a short story long (and I think I have), the
likelihood is that the design, length and diameter of the chain,
along with the properties of the pendant and it's bail are the
reason the chain does what it does.
Hope you had as much fun reading this as I did trying to figure out
how to say it