Yeah, it takes like a week tops to go from knowing nothing about private trackers to knowing what all the majors ones are. What.cd has been extremely well-known for a long time now. The exclusivity was less about actually being hidden and more about avoiding being the low-hanging fruit. What's been known to every government and anti-piracy initiative practically since its inception. It was just a matter of getting around to doing something about it.
Same way you know about it, I guess. An open secret really isn't a secret.