After the Southern Baptist Convention became a large and successful religion, modernists crept in who sought to turn them away from God and His Word. When the Convention refused to take serious action, many Baptists obeyed the Biblical command to separate, and they became independent.
Blessed by God, independent Baptists experienced great success in the 1960's and 1970's. But when they made the decision as a group, that sin in the leadership would be quietly smoothed over, things went wrong. They regarded themselves as "separated" because they still followed the man-made rules that hadn't worked, even while they disobeyed the Biblical command "Those that sin, rebuke before all." Losing the guidance of God because they didn't separate, many turned to Ruckmanism, regarding themselves as superior to Godly saints because they would only use the King James translation of the Bible.
Others, regarding Peter Ruckman as a wife-stealing enemy of Godly men, obeyed the Biblical command to separate. You'll find that independent, fundamentalist Baptists churches (often without the word "Baptist" is their name, though) still dominate the list of America's 100 largest churches. And despite their pride, Ruckmanites do not.
"Separation" does not refer to following man-made rules while tolerating sin. And Christians who tolerate sin don't get the guidance that God promises to those who separate.