Why does my paper airplane do this?

ok. so i made a paper airplane. when i go to throw it, i check it to make sure everything looks good. when its airborne, it will tatter side to side before it lands. it is extremely light and can go very far. i always flight it inside. no fans on or windows open. its the traditional design (but instead of folding the wing down, you fold the tip and then fold the rest of the side)(so it makes a pro wing on the bottom)) i do not know why it is tattering like that. help?