yup! - exactly - except a lot of the cards they used were stollen and belonged to random people.
It’s worth fighting those chargebacks. If the chargebacks from stolen cards were initiated by the real card owners, it’s very unlikely you’ll win the disputes. However if the chargebacks were initiated by the fraudsters themselves, that’s a different story and you might have a good chance of winning some disputes.
Did your platform have payout delays for new accounts? 14 days? Did you use 3D Secure payments?