I have no idea why he appeared to move the taser. Others have raised the possibility that Officers are required to keep control of their weapons, and that Officer Slager knew others were in the area and might have tried to snatch it.
I do not know how much these tasers cost (reported to be 1,000$ each), so he might have gotten in trouble for not regaining possession of it. Why he dropped it on the ground near Scott, and then picked it back up is unclear to me.
Someone else at the Treehouse wondered if Slager might have retrieved it because of the potential for damage by incoming police cars running over it.
Since I have no knowledge of police procedures, I see this as an opportunity for learning, and will be closely following what develops over at the Treehouse. I followed them during the Zimmerman and Brown mess of bad reporting, and found them honest, unbiased, and accurate.
Fine, move it but use a leatherman or something other than his hands.