1) Make a A/B test with an annoying pop-up sign-up box and an ordinary unobtrusive one.
2) Data shows that more people sign up using the pop-up.
3) Obviously people want to sign up, but can't find the unobtrusive box.
4) Hence, the pop-up design helps people achieve their goal (of signing up) better and is therefore more user friendly.
All of the things you described are not really any good UI or design patterns, but probably something from a "growth hacker", that were a/b tested and increased engagement metrics, so they got added. They didn't get added because someone saw them somewhere else, or the designer really wanted to add social media sharing buttons, 3rd party toolbars or interrupting the user with useless stuff.
This article is a welcome antidote.
This basically describes all of Babich's writing. He even names some of his articles "Popular Trends in..." or "Popular Techniques for...", but basically all of them are more or less a recap of what's trendy in one facet of UI/UX design.
Other examples:
* Social media share-buttons * Hard to find login links and just have prominent sign-up links * Extra 3rd party toolbars * Interrupting modal email sign-up boxes on a timer * Autoplay video
Each of these things (maybe) have a place, but there sure is a lot of "me too" going on as well.