One of the reasons was how the LXX translates Isaiah 9's messianic prophecy: "His name will be called, wonderful counselor"(you know the rest from Handel, doubtless!). Well, the LXX translate it as "His named will be called the Angel of Mighty counsel."
Why would they translate it this way? What I suspect is that the translators of the LXX are thinking of Judges 18, where Samson's parents encounter the Angel of the YHWH and they inquire regarding his name and he says his name is "Wonderful." The name wonderful means the "Angel of the YHWH."