what if it looked like there were multiple people? report of an officer that came up saw the shooter, was aimed at by the shooter and ducked out. is it possible that the USSS snipers weren't certain which person was the threat through the tree obscurance? possible that he came off the scope to view the entire situation to make sure that was the only threat and that there weren't more than one? you only come off the scope to get a full view of the picture.
i'm not confirming there was a cop near, just that there could be good valid reasons the USSS sniper had to come off the scope. there is heresay on faux that there was a cop near, so something like that could have been a valid reason to pull off the scope.