Yup after a while it just happens... Εspecially when different dj's are using them day by day. Most of them don't care...
Here http://www.ebay.com/sch/i.html?_from=R40&_trksid=p2050601.m570.l1313.TR0.TRC0.H0.Xcdj+2000+jog+&_nkw=cdj+2000+jog+&_sacat=0 you can find replacements and also protectors.