Slavery.
Don't be obtuse. They were telling them what to do about taxes. A guy (gal?) just posted Lincoln's quote upthread.
"As he said in his inaugural address: “there needs to be no bloodshed or violence, and there shall be none unless it be forced upon the national authority. The power confided to me will be used to hold, occupy, and possess the property and places belonging to the Government and to collect the duties and imposts; but beyond what may be necessary for these objects, there will be no invasion, no using of force against or among the people anywhere.” and ‘In your hands, my dissatisfied fellow-countrymen, and not in mine, is the momentous issue of civil war. The Government will not assail you. You can have no conflict without being yourselves the aggressors. You have no oath registered in heaven to destroy the Government, while I shall have the most solemn one to “preserve, protect, and defend it.”’
Lincoln repeatedly said before and after Sumter that he had no power to do anything about slavery. In fact, he would urge people to pass an amendment guaranteeing slavery.
Somebody as resistant to evidence as you are shouldn't use words like that. The argument was that the slave states wanted to leave because they didn't want Northerners telling them what to do. What was it that Northerners were telling them to do? Get rid of slavery. It doesn't matter if it was only a minority of Northerners telling them to do that. Their perception was that the North wanted to get rid of slavery, a fundament of their economy and society. All the rest was just excuses to cover for the real fear.