Read Christie Aschwanden's first paragraph. If you've written anything that elicits comments, I'm sure you can relate.
There is plenty of data here to digest, and also further speculation:
I had a hypothesis: Maybe this commenting-without-reading phenomenon represents a variation of the backfire effect, in which a person who receives evidence that their belief is erroneous actually becomes more strongly convinced of the viewpoint they already held. In this case, the reader sees a headline that catches their interest and reminds them of something that they already know, which triggers them to think about their pre-existing knowledge or belief about the subject and then to blast it out to the world. The article they’re reading doesn’t inform them, it just provides an opportunity for them to reinforce (and broadcast) what they already know.