I will never understand how people think a supreme judicial body with the power to hear whatever cases it wants for whatever purposes it wants, with the full ability to constrain or expand its decision to whichever procedural or substantive matters it so desires and bound largely only by a 221 year-old document and the things that the court itself has said can be non-activist.
Conservative judges are activists. Liberal judges are activists. You don't get a case in front of the Supreme Court unless a majority of the justices has something they want to say about the issue or issues in front of them. If the Supreme Court wasn't an activist institution, the only cases that would ever get heard were 9-0 clear findings of error on the part of the appellate judges. It's slightly clever branding, but otherwise an ultimately pointless label; to call any member of the Supreme Court "activist" is to call a member of the Supreme Court "Your Honor".