Here's the scenario, and I don't know what the answer is or why.

I have my old Martin in an open tuning (open D) and it is tuned two and a half full tones below concert. OK?

Now, when I finish the chord at the end of the particular song I'm working on, I hit the harmonics at fret twelve. That's when this really strange thing occurs. I take my hands off the neck of the guitar and just let the last chord (harmonic) fade. It seems that as it fades, it begins to rise a bit in key. Does anyone have a reason this would happen?