They let corporations submit blog posts (company spam, basically) and they'd instantly go to the top, pushing any normal submissions down into oblivion. Management thought that more ads will get them more revenue, but instead everyone just left.

It was a combination of factors.

They redesigned the website, which was a complete wipe of the service. All your submissions, comments, etc. Gone. That upset a lot of people.

They also changed their algorithm to prioritize paid promotional content over user submitted content, and they created a "power user" class whose content and liked articles would be more likely to reach the front page, too.