Checked CCBILL and our stats and everything looks fine.
We are all working in the dark and guessing. You need to get a couple of EXPERTS to help and give them access to your back end and your site.
They need to see what's the problem, when it started, where it's coming from and build a solution. Us guessing what the problem is, is no help to you.
If it just started or recent then you may have a fraud problem and you need to identify who sent the traffic. If it's long term then you need to address it and that will mean giving members easier access to get a refund and probably finding out why they asked for the charge back in the first place.
Stupid question but have you tried mailing the people doing the charge backs and asking them why?
We get 0.3% CB so never been a problem.