Mine does it too, sometimes at a trot. He is young though....is your horse young or just learning? All these are great tips. I am afraid to lunge in the bridle though as if he did turn and take off, I've heard horror stories of the bit doing damage to the mouth. A good tip I was told as well, is shorten your circle (make it smaller) so you have more control. If you're on a huge long line, it's easier for it to get out of control. Also, Make sure you are aware of his/her body language and look for the sign like he's going to spin and then you can pull his head in a bit and keep him moving in the direction you want.