- they removed downvoting ("burying")
- they took down the various "Upcoming" pages that allowed users to vet and weigh in on rising stories before they hit the front page
- they replaced the unified home page with algorithmic recommendations (and the algo was garbo)
- they introduced automated publisher accounts that circulated posts from official sources, crowding out organic submissions
- they deleted users' previously saved posts
- they largely deleted all posts, comments, vote counts older than a few months
- they broke the RSS feeds and third-party tools
All the above pissed off power users, and the rest were repelled by the nosedive in quality, the constant bugginess, and some terrible design choices (tiny light-blue-on-white text boxes, all usernames converted to lower case, impossible to view all comments, etc.)
My memory of the event was that they just launched a redesign that was half baked and everyone bailed overnight.
I've always thought that was why old.reddit.com still exists, paranoia about the same thing happening