Tribalism is definitely an underlying issue, but religion is directly connected to tribalism in this case. I'm not convinced America has a dog in this fight, but Christians do. A collapse of South Sudan would represent a victory for Islam, showing the Islamic world that even when Christians win a civil war, they can't govern themselves without the dhimmitude of Sharia.
For better or for worse, the South Sudanese are (at least relatively speaking) the “good guys” who spent decades fighting Islamic Jihadists who tried to destroy Sudan's Christian population. They're a mix of conservative Christians and animists who in both cases were subject to horrific genocide by Islamic radicals, along with some Muslims of black ancestry who the predominantly Arab Muslims in the north of Sudan didn't like for a combination of racism and belief they weren't sufficiently Islamist.
Some things are more complicated than they seem at first, and this is an example of a case where the “good guys” are not all wearing white hats. Those who have followed South Sudan for a long time knew that and were afraid something like this would happen.
The reality is that all of central Africa is becoming a Christian/Muslim battlefield. Sadly when the two cultures come in close physical proximity, there is no peace. Somehow the Africans will have to work out a consensus and modus vivendi. American mothers did not raise their sons to die in Africa.