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
Less tip more torque
Less torque more tip
More tip and torque
Tipping and retraction of anterior teeth