There are many different reasons why authors decide to self-translate their works. One can distinguish at least political, literary, economic and personal reasons.
André Brink started with self-translation because one of his work was banned in South Africa, so he decided to translate the original - written in Africaans - into English to get it published. Reaching a wider audience is also the main motiviation of authors writing in a minority language especially in Spain for Catalan and Galician writers but also for Gaelic authors. Some writers started to self-translate because they felt misunderstood and deceived by the translations done by professional translators. So they prefered to do it on their own or others like Kundera claimed to review the translations before they were published. Some bilingual writers explain that they feel the work is only completed if it exists in both languages. Several authors started to write the two works simultanously and translating helped them to find the weak points and so while translating they improved the other version and so the two versions were finished at the same time - before even one was published. Thats why often it is hard to tell which version was the first one or it doesn't really exist an "original" version. But this another issue I will soon adress in another posting. There are also personal reasons for self-translation, as some authors state that by chosing one language over the other, they feel kind of guilty. So self-translation helps them to restore a kind balance and harmony between their two languages. For others it is a way to stay in touch with both languages, especially if they live in exile, it is a way for them to keep their first language alive.