What's the motivation for this sort of attack? I'm quite sure chargebacks only ever put money back into the cards from which the money originated (i.e. the stolen credit cards; not into the attacker's account), which makes it seem kinda pointless.
Basically, someone farmed a bunch of numbers, sold it to someone, that new owner is now testing them to see if they can sell them further to others / launder the money off them.