Because you packed them into a tight clump then had them walk down a ramp while firing backwards up the ramp. Rogue collision volume is designed such that they can poke their launcher up out of a sea of Rogues on flat ground. But when firing up a ramp that they are walking down there are now Rogues high enough to be hit, walking towards those that are firing, all packed tightly together. A rocket will be fired in an empty space that is filled by a Rogue a few frames later. Accidents happen.