What causes triangular nostrils after rhinoplasty? (photo)

There are certainly technical issues that can produce this appearance.  There is only a certain amount of alar base reduction that can be achieved before nostril distortion can be an issue.  Find a board certified plastic surgeon who performs hundreds
of rhinoplasties and rhinoplasty revisions each year. Then look at the
plastic surgeon's website before and after photo galleries to get a
sense of who can deliver the results. Kenneth Hughes, MD Los Angeles, CA

You are correct. Rhinoplasty with sculpting the lower lateral cartilages, elevation of the tip with a septal graft, elevation of the caudal septum, and a alarplasty can create a triangular appearance to the alar rims. It may be more noticeable in noses that are originally wide in the alar base like african american noses, asian noses, and non-caucasian noses. However, it is seen in caucasians as well.Consult a Board Certified Plastic SurgeonBest wishes

Interesting question. Basically, it has to do with the anatomy of the lower lateral alar cartilages and the nostril rim... Best to seek in person explanations of the effect of rolling a triangular shape to an oval one-simple 3-D geometry.  

The notching that appears after some rhinoplasty is the result of over resection of the lower lateral cartilages or movement of the lower lateral cartilages medially and upward. This deformity can be corrected with repositioning of the lower lateral cartilages or placement of cartilage graft for added support or both.

With a lot of tip refinement/projection in thicker skinned noses, notching of the soft tissue triangle (what you call the triangular nostril) can occur due to an inherent tissue deficiency and scar contracture in an unsupported area of the nostril rim. This can be lessened by the placement of alar rim grafts or crushed cartilage grafts during closure in that area.

Notched nostrils is often a giveaway that rhinoplasty surgery has been performed.  It has to do with the skill of the surgeon and the techniques used in performing the rhinoplasty, not necessarily because of the goal of rhinoplasty (meaning it's not necessarily because of tip refinement or alar base reduction, etc)  If it does occur, it can be improved but it is difficult and there are limitations.  It is best to avoid it in the first place by seeking out a board certified specialist.