[F8] Why does the variable lose definition?

[dawsonk]

downM.onRelease = function() {
	_root.goalY = _root.myYArray[_root.i++];
	slideY();
};
upM.onRelease = function(){
	_root.goalY = _root.myYArray[_root.i--];
	slideY();
};
rightM.onRelease = function() {
	_root.goalX = _root.myXArray[_root.u++];
	slideX();
};
leftM.onRelease = function(){
	_root.goalX = _root.myXArray[_root.u--];
	slideX();
};