Treaty violations, mostly.

Here's a list of cases where the tribal authorities had to sue the US government, beginning in 1940: https://www.justice.gov/enrd/significant-indian-cases

Failure of the The Government to protect Indians on tribal lands: https://indianlaw.org/safewomen/violation-human-rights

The Government asserts it's sole right to prosecute crimes committed against Indians by non-Indians on reservations: https://www.bia.gov/as-ia/opa/online-press-release/solicitor-says-us-has-criminal-jurisdiction-reservations-where-tribes

State crimes are automatically Federal crimes on reservations: https://www.bia.gov/faqs/do-laws-apply-non-indians-also-apply-indians

A summation of American history with regards to removing the indigenous population: https://www.loc.gov/classroom-materials/immigration/native-american/removing-native-americans-from-their-land/

A US Government Accountability Office report on how the US is currently failing the people it sequestered on reservations: https://www.gao.gov/tribal-and-native-american-issues

Google what happened when the Sioux tried to have stricter Covid regulations on their reservation than the rest of South Dakota. The governor worked harder at stopping them then she did at stopping Covid.