Ethnic/racial, regional, and religious differences. For examples, see the rise of "white victimhood" type white nationalism in white america (Patrick Buchanan as an example). Certain regions are also becoming more 'black' - as shown in the return journey of black americans to the 'black belt' of the USA and black development of those regions (see Atlanta and Washington D.C. metro as examples). White flight from those regions also solidifies the territorial control. And of course, the Mormons still have been partitioned given the Republic of Deseret plan did not work exactly well, mormon regions outside Utah would want to rejoin Utah. Then there is aspects of ethnic cleansing in Los Angeles between black and hispanic gangs, or arguably death squads/militias now. And there are the ressurectionists of Texas and the Confederate states, and there is still a limited New England or now 'Northern Seaboard' mentality. Also, high speed rail linking Midwestern US cities will bring them closer and bring a sort of midwestern nationalism. Even now Southern Illinois still had sentiments to want to be its' own state, and there is a sentiment for places like the State of Jefferson & Franklin.
Now place this over a post-oil world, or a world dealing with Peak Oil or whatever (Yugoslav wars broke out during an economic recession), making large states harder to maintain - especially after nearly a century of cheap oil making interconnectedness easier. And add in the central government being perceived as ignoring black or white issues and grievances, or arguably suppressing nationalism by "polkitical correctness" and "false dialogues on race in america". Then add in Yugoslav perceived victimhood by both sides (Yugoslav peoples have plenty of recent examples, just open their history texts or talk to their parents and grandparents - something that works equally well in the USA). Funny as like the US, there were more people identifying as 'Yugoslav' even while the country was collapsing due to intermarriage between ethnic groups etc. The competing nationalisms, long suppressed as well as dormant hatreds reappeared.