I don’t know if the mystery of the handshake is still floating around or not but I will tell what I found out regardless.
So, I was running Fireteam matches with one of my designated team’s around last week and I noticed suddenly that always when we killed a Pred, defused and secured him. That it was our team leader and me doing the handshake in the cinematic.
My brain went like: “Wait a minute?” And while we were in the next match I checked what codenames we were assigned to. And it was him Alpha and me Beta, the assignments usually switched between us. But always he or me were one of those two codenames. And it was always me and him doing the handshake.
And that made me figure out that not the damage or who shot the Pred last before he died determined the handshake it was who was assigned Alpha and Beta.
So, if your not believing me. Try it out yourself. Find yourself a good team or friend and go an kill a Pred, defuse and secure him. And while your getting the handshake and you know which character is who. Check the in-game statistic for your codenames.
Hope you enjoy that information.