Linking to many articles and videos on the subject, Vox has a sort of meta-analysis of YouTuber's vocabulary and peculiar form of annunciation, which Mark Liberman of the University of Pennsylvania calls the "intellectual used-car-salesman voice."
In an investigation into YouTube voice, the Atlantic drew from several linguists to determine what specific qualities this manner of speaking shares. Essentially, it's all about pronunciation and pacing. On camera, it takes more effort to keep someone interested (see also: exaggerated hand and facial motions). "Changing of pacing — that gets your attention," said linguistics professor Naomi Baron. Baron noted that YouTubers tended to overstress both vowels and consonants, such as someone saying "exactly" like "eh-ckzACKTly." She guesses that the style stems in part from informal news broadcast programs or "infotainment" like The Daily Show.