Hey you're lucky! I never got the chance to fill out a survey from them ever in my life, maybe I'm just a Silver?
I don't how they pick the people to do the survey. But I just know that they have written names on the survey and they hand them out to those specific people.
I got invited at least 3 times when I was just a Silver... (I only became Silver a year ago and now a Gold...) both in Y and J... My guess is they picked from the MPC list on board, and also... I was waken in the middle of the flight by the noise or something and the FAs/Purser saw me being free so they picked me to do the survey? I tried to explain that I've done that several months ago already and they insist to have me do it again too... And i don't mind having another pen... and the chance for lucky draw which i have never won...
I still have two pens on hand now...
P.S. I was never a Green so am not sure if that matters for the odds