I know that neither of these phenomena originate in these communities. I just frequent both of them, and see them almost every day.
The mistaken belief that the predominant opinion of your favorite website's community is somehow a representative sample of some larger population. This is especially true if your favorite website's community reinforces, and enforces, the dominant opinion with upvote and downvote arrows.
The mistaken belief that you are very special, a thought leader, and therefore anything you think, or you notice, is definitely a trend or is otherwise representative of some larger population. An extreme version of the redditor's delusion. You should probably post your opinion somewhere prominent, without qualification, nuance, or perspective. Perhaps in the form of an assertion, if not proclamation. This is what is happening. You've noticed it.