It was probably the Reddit exodus. A lot of people from Reddit realized that HN has the same topics as Reddit and has the same upvote/downvote interface around the same time Reddit was having its API changes. The folks who did it were the kind who felt strongly enough about Reddit's leadership that they brought their strong opinions here.

Nothing you can do about open forums really. They all regress to the mean user who has enough time to spend hitting the up and downvote arrows on the website all day.