Botanists do know that the growing point of the cristate, or crested saguaro becomes multiple points which results in the fan-shaped top. Possible reasons include lightning strikes, freeze damage, or mutation. It is a rare condition - of the 1.6 million saguaros in Saguaro National Park, only 25 cresteds have been found.
The crested saguaro in this photo was inside the Desert Museum, a part museum, part zoo, part aviary (including a hummingbird aviary), and part botanical garden here in Tucson. Here are some more pictures from the museum: