I think the slide began with the advent of CNN and the 24-hour news networks. It was great at first, but after a while, they figured out the audience does not want to watch the same news over and over, and there's not enough breaking news occurring to have fresh content every hour. So, to keep viewers watching, they came up with shows like Crossfire, which was basically a few people sitting across from each other arguing. (They weren't the first to do this. For example, 60 Minutes had Point/Counterpoint and PBS had shows like The McLaughlin Group, both of which predate Crossfire. It seemed like CNN took the concept further.) The format is set up so that whoever yells the loudest and talks over their opponent is considered the winner, instead of the participant who presents the most cogent argument.
Another thing that seemed to me to compound the situation was the TV writers strike in the early 1990s. Because there weren't any writers to write scripts, networks embraced reality TV shows, which a) don't really require writers, and b) thrive on conflict. The producers just create situations designed to ignite conflict and let the cameras roll.
The media serves controversy, and the public laps it up.
Those two events accelerated the slide. Social media amplified it.