My cat does this too and after reading this thread, I am going to assume our kitties do this because they can't shed the nail sheath (sp?) as easily as they can the front. The front nails are easily shed on a scratching post, tree, carpet or your couch.
Oh yes! My cat does that all the time and it drives me crazy to watch her because she can really pull on those nails! It gives me the whillies to watch, lol, but it doesn't seem to harm anything so I guess it's just one of those little quirks they sometimes have!