I think the problem, at least in today's terms, started with the 24-hour news cycle. Before that, you got your news from the paper and the nightly news broadcast.
Now, they've got time to fill, and they have ratings to drive, and they all figured out pretty quickly that sensationalism and yellow journalism is the way to achieve that.
Where the news coverage used to be a whole lotta news with a few minutes of opinion, it's completely flipped, if you're following CNN/MSNBC/Fox.
Sadly, I'm convinced too many people can't tell the difference between news and opinion:
Biden Names Harris As VP Pick - news
Biden To Step Down If Elected - opinion
Trump Signs EO Limiting Muslim Travel -news
Trump Hates Muslims - opinion
Additionally, if you go out to hard left or hard right "news" sites, a quick perusal of the comments show that too many people read the always-misleading headline, then go straight to the comments and start vomiting their moral outrage there.
What happens after that is other people read that, then take the comments as news - they don't even read the story, and it's how you get untruths and overreactions passed off as news.
That's why, as an independent who falls on both sides of the spectrum depending on the issue, I have zero interest in extremists, whether they're right or left, even though I have what many would consider some extreme right and left views, depending on what we're talking about.