Just like Facebook and Twitter have done in recent times, Google is auditing the authenticity of accounts across YouTube.
A YouTube PM explained the decision today:
[Y]ou may see a noticeable decrease in your subscriber count as we remove spam subscriptions from your channel. We regularly verify the legitimacy of accounts and actions on your YouTube channel. We’ve recently identified and fixed an issue that caused some spam not to be removed. Today/tomorrow, we’ll be taking action and removing subscribers that were in fact spam from our systems.
Removing spam from the platform helps ensure that YouTube remains a fair playing field for everyone and should result in higher confidence that you’re organically building a community of authentic fans.
Google is advising users that it won’t affect their watch times, but users whose subscribers fall below the YouTube Partner Program will have to reapply.