There was a fancam in another angle (I only saw a gif of it though), you can actually see Hyoyeon's face, she was like wtf. And actually, in some fancam, it looked like she was almost grabbed too when he started pulling Taeyeon. I wouldn't blame her and I hate the fans who felt the need to blame the other girls for this. It all happened so fast and they didn't have fancams to watch over and over and over to analyse what was going on with her friend. She probably thought he was a staff since he dressed nicely. And the other girls? By the time they turned around, they were at the side of the stage already, with the MC too, they might have thought that Taeng left on her own.
I still believe that the guy isn't just some crazy fan, like that girl in vietnam who got up on stage and hug someone, but has some serious psychological problems.