Mine will usually spin one way just a bit untill the main rotor speeds up enough to counteract the tail rotor or vise versa. Your trim should be set to hold the tail at hover or flying speed, but when its sitting on the ground the main rotor isnt spinning at a high enough speed yet so the tail rotor speed is just a little off untill you wind it up and take off. It depends on how fast you increase the main rotor speed before lifting off, sometimes the tail will hold straight and sometimes it might float a little. At least thats what i have always asumed was the cause, made the most sense to me.