It was secession. Look it up. Anytime people break away and declare their own new country it is secession.
Or lack of law allowing such. In this case, the state constitution didn't grant the legislature the authority.
The state's constitution doesn't need to. After all, the powers of the states are almost too many to list out anyway. In The Federalist, no. 45, Madison said, "The powers delegated by the proposed Constitution to the Federal government are few and defined. Those which are to remain in the State governments are numerous and indefinite."
When they have to fight their way out it's called a rebellion.
The state's constitution doesn't need to.
According to Rawle it did. How can you point to part of his writings and say they prove your point and then ignore other parts and say he didn't really mean it?