Why Is This???

I could just be talking out of my ass here, but I think that has more to do with the actual vibration of the string than anything else.

 

i agree. its the attack/approach/idontknowthecorrectterm or how you pluck the string.

 

I might be talking out of my ass as well, but here's my guess on why this happens:

When you pluck a string, you initially stretch it, and in order to vibrate, it has to stretch a little for every vibration meaning a slightly higher pitch. As the vibration amplitude gets smaller, the pitch approaches its nominal pitch. The effect might be lower on instruments with high-quality bridges and nuts that probably are designed to compensate for this phenomenon.

 

I'm talking out of my mouth here, youre probaly puling the string too hard.

 

My guess who be that it appears to be getting higher, but really isn't. Lower tones require moving more air. As the note decays there is less vibration and less power to move air. As a result the bassier undertones become more less prominant, and the higher undertones become more dominant.

 

fretting too hard, or both.

Turn up, play softer, lower your action if you can.

 

Plucking hard is the most likely cause.