I think a lot of it has to do with how one acts, and that is not to say one must act suspicious to be picked. I was picked quite a bit on our last trip (02/09 - 02/16), and the majority of the time I was picked, I made eye contact with the security guard. Nothing menacing, just a friendly look, smile, and a hello. Could be that the security guard is trying to count the nth guest that enters, but with one security guard handling what seemed like the counting of 3 stations, I'm sure it can be difficult.
I didn't think anything was invasive. All the security guards I interacted with were kind and professional. They all seemed pretty happy, interestingly enough.
Around the 4th time I was picked, the security guard started to say, "Excuse me sir, you've been..." and I joined in with him on "randomly selected". And he asked, "do they get you a lot?" I said, "Yes, it must be the beard". We had a good laugh. He was appreciative that I was cooperative, I was appreciative that he was appreciative. Emptied my pockets (which actually got a pretty good laugh since I have my phone, charger, fiance's charger, hidden mickey book, wallet, poncho's, etc, in my cargo short pockets hahah), stepped through, they asked me what was around my wait, I showed them my belt buckle, and I was on my way.
It was all pretty pleasant to be honest.