I wet tumble everything with steel pins and love it. However, even if you are not using steel pins, you will noting a little "peening" around the case mouth. This is caused by the cases banging together while tumbling. However, I see this mostly on riffle case mouths.
My guess is that this "peening" might be creating a burr that is scraping the bullet as you are seating it, especially if you don't flair the case mouth enough.
My suggestion would be to chamfer the case mouth of a few of these cases and see if the copper shavings disappear. If so, then you can just flair the case mouth a bit more and the problem should go away.
I'm don't sure what dies you are using but I would suggest looking into purchasing a Lyman Type-M expanding die. These dies have a stepped expander plug that creates a larger opening for your bullet to seat without have to over-flair the case mouth. This style of expander plug is alos include in Redding and Dillon die sets.
Also, do you seat and crimp in the same step? If so, your die may be set incorrectly causing it to start crimping before the bullet is full seated. This will also cause shavings.
How this info helps.