We were also one of the few lucky couples selected for a ROC interview last year. We were completely confused as to why. We are both pretty young and never married before, we are the same age, both speak English, we both work, own a car, have 100% joint accounts, health insurance, etc. But, sometimes it happens.
Anyway, we were baffled trying to figure out what went "wrong" and how they were going to "grill us" at the interview. But honestly, we were in and out in 5 minutes. They asked the same basic questions we were asked at the AOS interview that is listed on the I-485 sheet. She asked each of us to share what the other person does for work and a few basic things like that. Our ROC was approved within an hour of us leaving. So, we still have no idea what triggered it. It definitely wasn't something we needed a lawyer for, and honestly, we never entertained it. That was our experience. Good luck!