Dorsal fin collapse can be caused by many factors including stress, old age, dehydration and fighting. For killer whales in captivity, the main reason for dorsal fin collapse is the swimming conditions.
The dorsal fin is made up not of bone but of collagen. Swimming puts pressure on this fin, which then helps to keep it straight. Wild killer whales swim deeper and farther than whales in captivity. As a result, fewer wild whales have dented or sagging dorsal fins. Whales in captivity are more prone to this problem because the swimming conditions are considerably different than those of their wild counterparts. They spend more time at the surface of the water with more time swimming in only one direction and less time swimming fast and deep. This problem can effect other parts of the whale, such as the tail flukes.
However, wild whales do occasionally show signs of dorsal fin collapse. According to About, a study in 1998 compared dorsal fin collapse in wild killer whale populations in New Zealand, Norway and British Columbia. Researchers found that killer whales in New Zealand had significantly higher rates of collapse, at 23 percent, than whales in the other two locations and speculated that this was due in large part to age and stress.