Wagon wheel effect is due to:

When buccal crown torque is incorporated in the wire, the roots of anterior teeth tend to converge towards centre like spokes of wheel. It has been often called wagon wheel effect. Following final space closure required torque is attained by incorporation of active root torque in the anterior segment. Due to wagon wheel effect, incorporation of palatal root/buccal crown torque during retraction causes loss of tip.   Orthodontics: Diagnosis and Management of Malocclusion and Dentofacial Deformities THIRD EDITION Om P. Kharbanda