EE was standing behind the operator while operator was setting the ATRS. The ATRS was almost to the top when a rock fell out hitting the ATRS and sliding down towards the canopy. The rock struck the canopy and bounced off striking the EE on the top of right hand causing a laceration that resulted in 6 stitches. The top was 14' high and the rock was 20""x14""x3"" about 20 pounds.