chargebacks submitted without customer knowledge?

I own a bar. Recently we've had 2 chargebacks come through -- one from an employee and one from a patron who comes in regularly. Both swear up and down that they did NOT file a chargeback. That goes against everything I understand about chargebacks, and seems like grounds for class action if true.

Has anyone else here experienced a credit card contesting charges without customer knowledge?

LOCATION: CA