When the Receiver of Memory dies or leaves, the memories the person had received are returned to the community. Since the community is not used to feelings of any kind, and has been sheltered from history, it comes as a great shock and causes turmoil and pain for the people.
In The Giver, one of the most prestigious roles in the Community is the Receiver of Memory. This role is taken by someone very special.
"But the Receiver-in-training cannot be observed, cannot be modified. That is stated quite clearly in the rules. He is to be alone, apart, while he is prepared by the current Receiver for the job which is the most honored in our community." (p. 61)
When the Receiver collects memories, they do not go to the people. So if anything happens to the Receiver and the person leaves, the community is thrown into turmoil.
"The community lost Rosemary after five weeks and it was a disaster for them. I don't know what the community would do if they lost you." (p. 144)
Jonas realizes that he carries a huge weight. Rosemary had asked to be released because she could not live with the pain. Jonas realizes that if he died, or left, the community would suffer.
"Rosemary had only those five weeks’ worth, and most of them were good ones. But there were those few terrible memories, the ones that had overwhelmed her. For a while they overwhelmed the community. All those feelings! They'd never experienced that before. (p. 144)
So losing the Receiver causes the community to experience the memories that the Receiver has. The community is not used to memories, especially pain, and it leads to chaos because the people do not know how to cope with their feelings.
Lowry, Lois (1993-04-26). The Giver (Newbery Medal Book). Houghton Mifflin Harcourt. Kindle Edition.