There are specific things which can force the changing of one's personality. I will address two specific disorders in regards to how the personality can change.
PTSD (Post-Traumatic Stress Disorder)
PTSD is the result of a a traumatic event. Although it gained much public attention in regards to soldiers returning from war, PTSD can also be the result of rape, incest, abuse, natural disasters, and accidents (such as car accidents).
In the case of PTSD, one's personality may change because of the things which trigger an episode. For example, if a person was not afraid of cars before an accident, he or she may lose control of his or her fears when around them after an accident. He or she may feel the need to shut himself or herself in a home in order to remove the possibility of triggering a memory of the event. In this case, the once outgoing person is now sheltered and a recluse.
In some cases, PTSD can stop certain developmental tasks from being accomplished. For example, if a child is raped, he or she may not adjust well to his or her sexual maturing.
With Alzhemier's, a person's personality can change dramatically. With Alzehimer's, people can become forgetful, have poor judgement, and confused. Their personality can change dramatically. He or she may remove themselves from social activities (which they previously enjoyed). They may also become depressed, anxious, fearful, and suspicious.
In regards to developmental tasks in Alzhemer's, this is normally associated with the elderly. In this case, he or she will have forgotten how to perform simple tasks developed earlier in life.