Automated passport kiosks, deferral with ER using Green Card

I am a recent green card holder, obtained from work/EB2 category (if that makes a difference). The first time I used it I was traveling by car, and it was very straightforward. The second and third times have both been entering the U.S.A. via DFW International Airport. Both times I used the APK and have received a referral with code ER which seems to mean "Enforcement Referal". After being referred both times there were no questions asked, but they asked for my green card and passport. I did a 4- finger print, a photo and they stamped my passport with entry "ARC" (not sure what this means exactly?), took less than 1 minute.

However I've been wondering why I am getting referred, both times it has been the same flight American Airlines from Mexico City, and I have noticed almost everyone who used the APK from the same flight got referred with me, all green card holders.

I read that it could be because of any minor criminal offenses, but I have never been fingerprinted for any type of offense at all.

I was wondering if I could ask the CBP officer next time, or if I could contact USCIS to inquire.