image moves in viewfinder with Sigma 17 50 !!!

i have a d7000 i noticed sometimes image moves in the viewfinder when the os is on with the sigma 17 50 specially when zooming to 50 , this doesn't happen with the 35 or the 18 105 or the 18 55 is it normal ? knowing that the lens has zoom creeping when looking up it goes down when zooming to 20mm .
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