We'll let Boogie tell you about the details, but the TL;DR version is this: thousands of copyright claim notices have gone out to video game based YouTubers on Let's Plays, reviews, and commentary videos. Basically, any video that uses game footage is in danger. Many game companies don't allow footage to be used in monetized videos, and some don't allow any footage at all, and YouTube is enforcing those claims en masse. A few companies/games have come out in support of YouTubers:
Good on you, Diablo. What do you think about Copyrightalypse?