I recall learning about this in my undergraduate studies. In 1823, there was a Supreme Court ruling that determined that a tribe’s “right to occupancy” was overridden by a U.S. Citizen’s “right to discovery”. The Right of Discovery is an old old old “doctrine” from 1095 (crusades!) that essentially allowed Christians to discover–or claim–land from Non-Christian territories. The 1823 Supreme Court ruling that sited the Right to Discovery was in response to a period of “voluntary” Native migration and the numbers of people moving was simply not high enough to satisfy the US Government. The ruling “allowed” (quotes intentional…) natives to remain on their land, but it prohibited them from holding the title to the land that had been “discovered”. No title, and they then become squatters when the land is owned by someone new.
Several of the Nations were attempting to work with the government at the time in order to retain some of their land, but bit by bit more was taken away from them.
There is of course much more to the story, but this is what I was able to recall from my studies. I did a quick search to pull up some references for you, but I will confess that I did not read them through terribly thoroughly!