It has to do with us noticing an uptick in two undesirable things: harsh ideological comments, and accounts that use HN primarily for political battle.
You can see one example of how the idea developed here:
That's straightforward, and understandable.
Personally, though, I would want to measure that in some way. We all tend to notice harsh comments or politicized accounts when they are in opposition to our world view. Ones that we agree with might fly under the radar.