It was a bug in the subscriber count system.
It was a bug in the counting system. People would unsubscribe from the channel but the number wouldn't go down, in some cases. When they found the problem and fixed it, the numbers went down all at once. Everyone freaked out.
Jacksepticeye explained in his video (starts at 2:03) today. He said it was a bug, but the rest of the explanation didn't match your version. He said that youtube usually deletes inactive accounts on a regular basis, so the number goes down a little at a time. The program deleting the inactive accounts had a bug and didn't work, so when they fixed it, all the inactive accounts and spam accounts were deleted at once. Jacksepticeye had 260K subs deleted. From his version of the issue, the unsubscribe function was still working. Do you have a source for your version?
Pewdiepie made a parody video about the "bug". He lost 600K subs. If it was a bug, his subs didn't rebound. He also mentioned youtube writing something about inactive or "spam" accounts but I couldn't read what he was reading because his version didn't look like it was an English version although it went on the screen so quickly.