It was explained that we stand by kiddush because of testimony. Why do we stay standing after the first paragraph? Why do some stand for day kiddush?
Once we started the Friday night kiddush standing because of the testimony, some say it's logical to continue the rest standing, as well. There's a kabbalistic reason as well, to stand (Shabbat is likened to a bride, who's blessings are recited standing), and there are those who apply this reason to stand for kiddush in the morning, as well.