Why taking it off makes that happen I have no idea.
Adds weight, stability and levels COG.
My best guess would be that the gyro is set to compensate for yaw taking into account the weight of the tail. Without the weight back there it overcompensates and twitches. Turning the motor gain pot down might correct that.
Hance,
Are you at significant altitude by any chance? I'm curious whether the Blade 400 will do well up here.