The leaving state walks away from the duties and obligations that the country as a whole entered into while that state was a member. That can have a negative impact on the interests of the remaining states and such fairness alone would require that those issues be addressed and settled by mutual consent before the seceding state leaves.
Why before? Is that because the state would be at the mercy of a federal court, which we both know would be the ultimate arbiter? Why not after, when the state can negotiate on a more equal footing? And realistically, the seceded state(s) would still need a harmonious relationship with the U.S., so an equitable arrangement would have to be worked out in the view of the state, so as to keep trade and other activities going.