Posting in ALL CAPS, posting fake news or insults to other members of the community, posting to incite anger, etc. does not make for good discussion, and hurts the sense of community. That, and bringing that to other subreddits via brigading, etc.
They modified it before the election (during implied they did it and then put it back, and unfairly implies it was because they wanted to "suppress the truth" or whatever), and to be clear it was to combat the fact that the_donald readers upvote all their posts with a huge amount of fervour that doesn't align with other subreddits.