I find that I slip into the same habit sometimes – specifically, when I’m typing something where perfect grammar will come off as overly formal, but I’m stressing a bit over what level of informality is appropriate. It taxes the frontal lobes, which means that my thoughts are disjointed in a way that ellipses just make sense with. I think it’s so widespread among older people because when they were first introduced to instant messaging, they weren’t sure how they would come off, and then it became a habit. But that’s just me guessing.