The sternocleidomastoid muscle helps turn the head. The two muscles working together regulate the position of the face - straight or off to the side. Treating it with Botox can potentially cause a decreased function of one muscle, causing a deviation of the head from its central position.
I must say I wonder about the sudden number of questions about using Botox to decrease the size of large muscles. And as many questions as there are about this topic, there are many more answers talking about the danger and the inadvisability of performing such a procedure.
Using Botox on large muscles would require large quantities of Botox which can potentially be dangerous or even life-threatening. Also, it may induce antibody formation to Botox, and since Botox use is needed every 2-3 months to maintain results, it will be useless with subsequent injections. Please don't do it.