There have been thousands of accounts that stole names and content of others pretending to be these people (identity theft + plagiarism). Since the account wasn't verified, it was likely suspected to be one of these cases where YouTube user is being impersonated.
Did they give a basis for the claim, or reason to believe it was theft/plagiarism? It's an odd thing to do for no reason.
There have been thousands of accounts that stole names and content of others pretending to be these people (identity theft + plagiarism). Since the account wasn't verified, it was likely suspected to be one of these cases where YouTube user is being impersonated.