Flag of the Independent Ogalala Nation.
By Tripodero - Own work [1], CC0, https://commons.wikimedia.org/w/index.php?curid=65312096
The occupation grew out of protests on the Pine Ridge Reservation regarding tribal government, something that's generally been forgotten about the dramatic occupation, with its immediate cause being the failed impeachment of Tribal Council Chairman Dick Wilson. It spread to a larger set of grievances soon thereafter, but Wilson remained the center of the controversy in significant ways. He'd be reelected in 1974, although the reelection was controversial, and the reservation, which remains the poorest reservation in the United States, endured a period of violence thereafter.
Poster made from one of the photographs of Bobby Onco.
I can recall this event fairly well, even though I was ten years old at the time. One of the things I oddly recall is discussion of one of the occupiers being armed with an AKM (AK47), something that was extremely unusual at the time. The man in question was Bobby Onco, who died in 2014 at age 63. Onco was actually a member of the Kiowa nation, and was a Vietnam Veteran. Perhaps surprising to many now, it was possible to bring captured weapons back from Vietnam, with some paperwork being required in order to do it. The iconoic Soviet assault rifle was likely a legally returned weapon from his Vietnam service.
Onco would move to New York, where he married a member of the Shinnecock Nation. He lived on their reservation there until his death.
The occupation was not universally well received by residents of Pine Ridge, which is part of what ultimately brought it to an end. From the outside, but in the region, it was one of the events that gave the late 60s and early 70s the feeling that things were coming apart.