The one thing upon which most Americans seem to be in perfect agreement is that the nation has truly never been more divided than it is now. Any cause for a civil war would certainly depend upon what side of the political spectrum you fall. For the sake of being perfectly objective, this response will attempt to take neither side and answer only what causes would lead to state's succession. However, those reasons will be quite different based on whether the succeeding state were predominantly democrat or republican.
In the case of a red state succeeding from the United States, the cause would most likely be the usurpation of what they consider to be constitutional rights. One of the most common issues that draws out threats of succession is the issue of gun control. In the past decade, the United States has seen attacks from domestic terrorists with the use of combat rifles in numbers that are unheard of in any other part of the civilized world.
This has been met with overwhelming demands of more rigid gun control from the most minimum of background requirements to all-out bans, similar to those maintained by the United Kingdom. Red states tend to see this as a blatant violation of the second amendment, and they show animosity toward the idea of a government disarming its people. In fairness, this has been a historical ill-omen in many parts of the world.
In the case of a blue state succeeding from the United States, a probable cause would be impatience with the slow-going social reform that they consider to be integral to a modern society. Liberals and leftists are primarily concerned with the healthcare system in the United States, which they consider to be largely inferior and even predatory toward sick persons of a lower class.
In this regard, rampant capitalism is also a concern, as they maintain that lobbyists and corporate welfare render a government impotent, and that titans of industry become the de facto power in a country with this type of free market. In fairness, it is quite surprising to learn that Amazon, a company that made $11.2 billion in profits in 2018, paid absolutely $0 in taxes and is reported to have employees that are living on government welfare. The lack of security for workers and wealth disparity are great concerns to the left, as they maintain that a government that has the resources to take care of its people but does not is no true government but is rather a tyrannical institution.
If a war were to occur within the United States, the cost would be the lives of innocent people throughout the entire country. We are not a nation that has known war on the home-front in nearly a century, and the bloody conflict would likely break us as a people forever.