Elizabeth Freeman, or "Mum Bett" was famous as an African-American woman who was bold enough to stand up for her rights and for those of people around her. When she died in 1829, she was generally believed to be 87 years old. Even today, that is certainly a ripe old age. In those days, it was even more so. Therefore, it is safe to say that Freeman died of natural causes -- of simple old age.
Freeman is best known for her involvement in the court case of Brom & Bett v. Ashley. In that case (1781), she won her freedom by suing under the constitution of the new state of Massachusetts (after the Revolution). She is, therefore, famous as an early leader among African Americans in standing up and demanding her rights.