During RIH 9 5/8” casing, Roustabout-2 wrapped the webbing sling on the casing joint from V door side to the casing which was rested on the two pipe racks. The Crane Operator moved the crane boom in order to give adequate slackness to the sling, for proper wrapping of the casing in the catwalk side. This move caused the casing to slide towards to the end of the catwalk. Roustabout-1 was standing next to the landed point between catwalk & pipe rack.