WORRIED! Recieved RFIE and don't know why

Our I485 and I765 docs were sent and received on November 8th. On December 20th we had the biometrics, and today when checking the status it has Request For Intitial Evidence was sent. My wife came in on a K1 Visa and we have been extremely careful to do everything correctly. During K1 process we never had a RFE so this has never happened to us. We sent literally everything and spent countless hours watching videos and going through checklists. We even paid for a new medical so we had fresh documents and everything. We can check the docs page online since we sent the case in paper. Is this extremely common to happen? We sent all of the below for the I864:

○ Completed and signed form I-864 ○ Copy of sponsor’s proof of status (U.S. Passport, birth certificate) ○ Copy of sponsor’s tax returns and W2s/1099s from 3 most recent tax years ○ Employment verification letter stating current annual salary

Is there any ideas on what this could be about? She hasn't worked in 6+ months....