Its continuing existence, not the establishment is relevant. It was also established, for example, in New York State but New York State did not have cause to secede in 1861.
Furthermore, slave trading was done to meet a demand. Remove one set of suppliers but keep the demand and it would still exist
It's origin, not it's continuing existence, is the critical point. Again: no slave-trade=no slavery=no secession (according to you). Therefore slave-trading is the real (root) cause of secession.