It is true that Ashkenazi Jews and Caucasians of Northern European descent are more likely to be affected by CF (1/29 are carriers). Basically, at some point in evolutionary history, a genetic mutation was added to the gene pools of these two populations (possibly around the same time in history). Since it is more common for people to reproduce within their own race/ethnicity, the disease will be perpetuated at an accelerated rate within those specific subpopulations, especially in the Ashkenazi Jewish population which also has a higher frequency of other "rare" diseases such as Tay-Sachs Disease. However, you should note that the mutations causing CF are probably different point mutations in the DNA. Therefore, the "cause" of CF in these two populations are probably different.