The hackers used phishing to steal credentials from YouTube channel owners. Many owners said the hackers sent emails that lured them into phishing sites that looked like Google sign-in sites. After the owners entered their YouTube credentials, the attackers used the details to sign in to Google Accounts and assign the channels to new owners.
You also changed the channel's "vanity URL" to be similar to the channel and the account was deleted even though the channels are available for sale in some Darknet forums.
The phishing emails hackers use to attract potential targets are so well-crafted that they look real at first glance.
ZDNet is a primary concern of developers that hackers can bypass two-factor authentication for user accounts. One victim said the cyber criminals might have used a Reverse Proxy-based Modlishka Phishing Toolkit to reroute these attacks, assigning accounts to a new owner as YouTube will soon restore all the hijacked accounts and return them to their original owners.
Askamani is active in OGUsers, a darknet forum used by hackers to sell access to hacked accounts, including YouTube.  Askamani also said that an increase in complaints would mean that someone gets hold of a "really nice database" and that he now gets "a bang for his money".