I think it's a degradation similar to the way in which mirror bumper foams and light seals go gooey over time.

Like a previous poster I first noticed it with a Sigma zoom (which was in pretty nice condition apart from the gooey grip), the Pentax SF7 bundled with it had suffered a similar problem with the plastic eyecup/viewfinder trim moulding where it had begun to "sweat" and crack up. As it was only 7 (bought for a party where I wanted something automatic but cheap, it ended up shooting Ilford Delta 3200 with a Pentax-M 50mm f1.7 on the front which had made a full recovery from a serious mould infestation) I wasn't too worried.

I initially thought that it was an enviromental thing and the lens had somehow damaged the part on the camera, but then I noticed my other SF7 showing early signs of the same problem. The rest of the camera is absolutely fine in both cases, I removed the eyecups and used lens cleaning spray on a cotton bud to clean the goo off.