I didn't even know bullets can ricochet off a dirt or sandy berm. I guess it's similar when we skip stones off the water surface? But bullet configuration vs. the flat sided surface of a stone......
A shot aimed in the air makes sense to me. But if that was the case, why are they re-building the berms?
Sorry gang, just thinking out loud. As it was mentioned, I should just shut up and wait for the official report and announcement scheduled for today. 
The bullets are supposed to all go directly into the dirt backstop which would stop the bullets. However there are riccochete hazards when they don't do that. Many reasons...
1. Backstop not maintained. Bullets will pile up inside the backstop if not regularly removed, and bullets fired into the mass will ricochet.
2. 50 Yard target. Bullets will hit the ground at a shallow angle and some may skip over the backstop. Which is why they pile sand on the ground to catch the bullets.
3. Steel tarets, frames, rocks, other objects. Bullets can deflect off objects. Skim the top of the frame and go over the backstop. Hit the edge of a frame at 50 yards deflect to a steel target at 100 to the side, then deflect off a the steel target rebar legs back to the firing line.
4. Cratered steel plates. Shoot a crater in a worn steel plate could result in the bullet or splatter coming back to the firing line.
If something has a one in a million chance of happening, and you fire a million bullets, that chance will happen.
There's a reason for the range rules at Koko Head though they may not be apparrent. One of the range safety improvement ideas was to bulldoze the elevated rifle firing positions and level it flat, so that the shooter, 50 yard target, and backstop are in line to prevent ricochets.