In my mind, there are two pretty straightforward, first principles at work that underlay a lot of these items.
Many, if not all, negative emotions, at least while being experienced, directly diminish cognitive capabilities. A concrete example: when a person is angry, that person is more stupid.
Next: modern communication technology allows/facilitates/encourages negative emotions to be generated, travel widely, and 'stick' in the minds of more and more people.
As I said, there's a lot of good analysis, but I believe that these two simple things are the most responsible for 'stupidity expanding'.
I went out of my way to state these things briefly, because fundamentally I think they are simple. At the same time, there are mountains of nuances and relevant conditions surrounding them.