That's a co-surgery type denial. Thatwould normally be seen if a 62 was appended and both MDs are of the same specialty.
Either these have a 62 modifier incorrectly where it should be 80, the codes do not allow assistant at surgery or co-surgery, or there was a claim issue and/or processing error with that MAC which you would need to look into. If it was Medicare primary of course Anthem secondary would also deny. If Anthem was primary and doing the same thing it has to be an incorrect 62.
It could be incorrect denials from a payer, that does happen. Have you contacted Novitas? You'll have to have someone figure out the impacted claims, time frame, and codes, etc. and compare to other claims outside of the time frame with the same codes and providers to figure it out. It could also be something with a provider's credentialing. Are claims with these providers in other time periods, paid, etc?
I have seen where a practice management system is incorrectly updating/flipping modifiers and other items like changing a CPT to a G code or auto-changing items on the claim but it looks correct when entered by a person into a billing screen.
If your group is that big, you should have a data analytics person or someone higher up in the rev cycle that can help with this. They would need to know things like the # of impacted claims, time frame, providers involved, the codes, and the exact denial reason/remark code. If it's that big of a problem someone in the rev cycle higher up or the provider would have noticed depending on how long it's been going on. It sounds like if you are a staff coder, maybe you need to bring it to a supervisor or someone higher up for team help. When you say, "insurance is denying claims. They are all coming back denied" what does "all" mean? It needs to be quantified. If it was all of their surgical cases they would be having a freakout, especially if their paid by RVUs.