I thought causes of death were supposed to be public in the US. Is there any way to know for sure why she died?
Governmental disclosure of information on death certificates is controlled by state law. In Washington, vital records are controlled by RCW 70.58A. §520 limits the possible recipients of such records. One category is "for research", which involves a review that covers guarantees of confidentiality among other things. §530 allows certain other disclosures, specifically to
(a) The decedent's spouse or domestic partner, child, parent, stepparent, stepchild, sibling, grandparent, great grandparent, grandchild, legal guardian immediately prior to death, legal representative, authorized representative, or next of kin as specified in RCW 11.28.120;
(b) A funeral director, the funeral establishment licensed pursuant to chapter 18.39 RCW, or the person having the right to control the disposition of the human remains under RCW 68.50.160 named on the death record, within twelve months of the date of death; or
(c) A government agency or court, if the certification will be used in the conduct of the agency's or court's official duties.
(10) The state or local registrar may issue a short form certification of death that does not display information relating to cause and manner of death to a qualified applicant. In addition to the qualified applicants listed in subsection (9) of this section, a qualified applicant for a short form certification of death includes: (a) A title insurer or title insurance agent handling a transaction involving real property in which the decedent held some right, title, or interest; or (b) A person that demonstrates that the certified copy is necessary for a determination related to the death or the protection of a personal or property right related to the death.
Then finally, and this is as close to a "public information" clause that you get,
(17) The state or local registrar must issue an informational copy from the central vital records system to anyone. Informational copies must contain only the information allowed by rule. Informational copies of death records must not display information related to cause and manner of death.
Vital records are specifically exempted from the Public Records Act.
California also allows disclosure of a subset of death certificate information. The records are exempt from mandatory disclosure under the California Public Records Act, but the registrar will release "noncomprehensive death record indices for public release shall be comprised of first, middle, and last name, sex, date of birth, place of birth, place of death, date of death, and father’s last name". One would first have to determine what state a person died in, in case that state has fewer restrictions on disclosing personal information.
Because Ms. Charles was in New York, her death certificate is not a public record under Section 4174(1)(a) of the Public Health Law, and the report of any autopsy that may have occurred is also exempt from the state's Freedom of Information Law. In re Turkel, 236 A.D.2d 283, (N.Y. App. Div. 1997).