Seems like PewDiePie experienced a horrific flashback after recently completing the playthough.
Sources:
PewDiePie: https://www.youtube.com/watch?v=qhu7Xhrt2dE (if the video is still up before it gets removed by copyright)
Quiet's Theme from MGSV Soundtrack, cover by Theishter: https://www.youtube.com/watch?v=lnWib0CGBT0
Character Sprites from Doki Doki Literature Club!
