I don't think many if any of the things revolving around the exact "dream nature" are explained through some scientific means. But seeing how dreams often reflect thoughts and experiences gathered through the day or just in a general longer timespan, it might be interesting to hypothesize various reasons specifically through that lens.
I don't have 3rd person dreams too often, but some sort of spectator like experiences do happen. One could attribute that to the way TV or movies work, or should I say the way they could be percieved by the brain. Video games could also be a culprit. Being interested in the whole mind-brain relationship through introspection for a long time now, I must say that I do recognize for myself, how the brain extends its perception and influence to what could be called "virtual experiences" like TV or video games. For example when sitting down to do something on the computer in a dream, often I don't just stay in front of the computer with my hands on the keyboard, but rather the screen and whatever I'm doing, takes over the dream completely - vision, audio, perception of a body disapears etc. That's one way I find these strange points of view come about, for me of course.
Another reason could perhaps be that this is the way "you" sometimes think about yourself. When I was younger I used to remember things in 3rd person from time to time. Don't know why, but it happened. They were sort of false memories of course, since my eyes didn't float outside my body to take pictures, but still genuine. So even now if I try to remember something generic from my childhood I remember some events just like that. Do you have something similar going on?
Dreams just differ so much from person to person it's really hard to generalize something like this and find the exact cause. What kind of themes, mental attitudes or emotions do you notice in 3rd person dreams? Anything special about them beside the point of view?
Just my 2 cents I suppose.