Question

What was the immediate cause of secession of southern states from the union prior to start of the civil war?

Answer 1

the conflict between Union troops and Confederates at Fort Sumter

Answer 2

The fact that Abraham Lincoln won the election even though he was not even on the ballots in a lot of southern states made the southerners feel like they did not have a say, which is why South Carolina left the union (the other states were relatively quick to follow).