It happens occasionally on almost every line. Frequently the reason is that the ship is chartered by a corporation. Sometimes it is a mechanical problem that requires attention (such situations usually receive a lot of publicity), and sometimes a revision to the ship's schedule or itinerary is responsible. Since 9/11 there have been a number of itinerary and schedule changes, and many of the lines have announced that they will be sailing from new ports in the coming months. Any of the above reasons could explain the cancellation to which you refer, but without more details anything we can say would be pure speculation. Cruise lines will usually offer you several alternatives such as: sailing on the same dates on another ship or other itinerary- if available, substituting another cruise at a future date (usually with some time limitations), or a refund of your payment. I am sure your friends are disappointed but I hope that they and the cruise line can come up with an acceptable alternative.