There are many reasons the tip of the nose is crooked. They include a fractured or warped septum, a dorsal irregularity of the bridge of the nose, inwardly deviated upper lateral cartilages, and an entire nasal bone fracture. The septal spine can also be fractured and deviated creating a twisted columella. A septoplasty is an operation to straighten the septum, but on many occasions other additional surgery may be employed, such as a tip surgery, spreader grafts, osteotomies, which are all considered more cosmetic in nature.
Web reference: http://www.seattlefacial.com