"So what was their beef?"
What was the beef in them leaving if they had such control then?
The South wanted out. If you're going to ask me what specific cause each person wanted out FOR I couldn't say. Aside from freedom from the Fed, I have no answer. And I'm afraid noone can have an answer to it.
If the South had no justification for revolution, they had no right to leave a Union they had agreed to.
Well, many of the seceding states did issue "Declarations of Causes," but the Confederate side of the debate tends to downplay those as the private opinions of a few people because they explicitly state that it was all about preserving slavery. The fact is that there are loads of documents and speeches from the south saying that secession was an attempt to protect slavery, but these are always discounted.
http://www.yale.edu/lawweb/avalon/csa/scarsec.htm
http://www.sonofthesouth.net/leefoundation/Georgia_Secession_Causes.htm
http://members.aol.com/jfepperson/reasons.html