I'm gonna try to do this from memory because I don't have the time today to research. I'm not saying this is the for sure reason. It is something I hold in my mind as a possibility.
I believe it is called the "Middleman Minority Theory."
What is a Middleman? A Middleman is someone whose career does not involve producing something. A farmer is not a Middleman. A carpenter is not a Middleman. The guy on the auto assembly line is not a Middleman. Rather a Middleman would be the one who makes loans, or a trader who ships products from one place to another.
When Middleman industries become dominated by an ethnic minority, that minority becomes vulnerable to vicious conspiracy theories, and the society seeks to get rid of them in some way or other, either by rendering them completely powerless politically. exiling them, or killing them. For example, Idi Amin expelled all the South Asians (whom he referred to as brown Jews) from Uganda. Other examples include the anti-Chinese riots in Indonesia, the Armenian genocide, and sheesh I just can't remember others, it was a very long very extensive list. I stumbled across this in one of my many efforts to try to get a handle on antisemitism.
Anyhow, the Rwandan genocide was one of the genocides listed that fit the Middleman Minority pattern.