From the National Park Service website:

Even when saguaro cacti grow in their normal form, they rarely grow symmetrically. Saguaros sometimes grow in odd or misshapen forms. The growing tip occasionally produces a fan-like form which is referred to as crested or cristate. Though these crested saguaros are somewhat rare, over 25 have been found within the boundaries of the park. Biologists disagree as to why some saguaros grow in this unusual form. Some speculate that it is a genetic mutation. Others say it is the result of a lightning strike or freeze damage. At this point we simply do not know what causes this rare, crested form.

We were fortunate to see a few of these rare cacti during our time in the park. We saw the nicest specimen on our hike down the Garwood Trail on day three.