One of the major causes of death among captive dolphins is suicide. They kill themselves in many different ways from just refusing to eat all the way to the extreme of smashing their heads against walls!
Dolphins sometime have trouble adapting to new ways of life. For instance, dolphins mainly rely on raw fish diets that they hunt for themselves. When in captivity, they are fed pieces of fish from humans, which can shock the dolphins and send them spiraling into suicide.
Dolphins die from smashing their heads on the walls, but it is not always intentional! Many times, the dolphins will get excited and not understand the boundaries of the tanks they are in, so much so that they accidentally continually crash into them! Read more about this phenomenon by clicking the source!