Happens to me as well - in fact, just the other day. I was fighting against this girl in a hotel lobby in my dream and stats combined, I should have won, but as we fought she seemed to get stronger as I grew weaker and in the end an objective third party ended up seperating us and sending the other girl to bed.

I guess it depends what the fighting in the dream is meant to represent too. If it's some kind of a reflection of the outside world and troubles with others, I also wonder if part of it is you feeling helpless on some level to overcoming whatever disharmony with hypothetical other person you feel.

Though - as stated above - I imagine gravity (lying on your back) also has a significant part to do with this too. As does your body's natural paralyzing effect on you as you sleep to protect you from injuring yourself...

Interesting topic