Earlier this year, YouTube announced changes to the current Subscriber count system, but the complete process had yet to occur.
So, many YouTube channels may notice unusual drops in Subscriber counts. This is part of the previously mentioned change. Closed accounts are being removed from the public Subscriber count, meaning that many of the drops are not from actual unsubscribing users.
A more accurate count of how many Subscriptions/Unsubscriptions your YouTube channel has can be found in YouTube Analytics.