I have no idea.
This shot was with a P25 back. I've had it happen on my DSLR also, except the lines are horizontal in that case.
I used to think it was the card or the card reader, but I've eliminated that possibility. The original RAW file as well as the LR generated DNG file both exhibit this, so I'm guessing it isn't LR. I copy the RAW to the hard drive and then do my import from there, never from the card directly.
Because this has now happened with two different cameras I'm also eliminating the sensor itself as a cause of this. There's got to be some component in my workflow that responsible, just don't know what.