The right to have their votes counted, for one. A guy who didn't even qualify for the ballot in most of what became the Confederacy was foisted upon them by the rest of the country. THAT was the last straw. The fight over slavery had been going on for at least a decade, if not more. Yet it was only after the 1860 election that the South had enough and walked out.
Yep'
Nominees Electoral Vote Popular Vote
Presidential Vice Presidential
Republican winner Abraham Lincoln Hannibal Hamlin 180 59.4% 1,865,908 39.9%
Southern Democratic John Breckenridge Joseph Lane 72 23.8% 848,019 18.1%
Constitutional Union John Bell Edward Everett 39 12.9% 590,901 12.6%
Democratic Stephen Douglas Herschel Johnson 12 4.0% 1,380,202 29.5%
http://www.presidency.ucsb.edu/showelection.php?year=1860