I used to have the same problem. But it just faded away through time. I don't know what it is, it just went.
A little theory of mine is that, if you are right handed, the muscules are used to more complex movements. Playing octaves, your hand remains in the same position for quite some time. When I hold up both my hands in the air, like, above my keyboard right now, it's harder for me to keep my right hand still than to keep my left hand still. Perhaps because the hand is used to more complex movements, it's harder to keep it still, so it would also be harder to keep into octave playing shape, which creates the tension. So that means the muscules in your right hand have to get used to keeping your hand in the same shape for quite some time. This should, if I am right, be otherwise for left handed people.
But ofcourse, this are only just guessings.