Volume of fire.

500m (1640ft) max effective range, estimated

large and open target-rich area with extremely limited routes of egress

It seems the Fish-In-A-Barrel analogy would be appropriate, no matter where the shooter(s) chose for their vantage. Although an elevated spot would've been far more effective than present at the target site. A shooter on-scene would be less effective, and have less materials for times between salvos. Also, bodies at standing height would stop projectiles before they could hit targets more distant (look for studies on the fragmentary effects of IEDs like vest bombs; three persons out would not expect to be penetrated by shrapnel)

The 5.56 does some crazy turns once it hits bone (or belt buckles, wallets, pacemakers, a P-38 can opener, etc.)

It wasn't full auto. Depending on the ammo and barrel you can easly shoot 556 nato 1000 yards. he was in an elevated position shooting at a large group of people. A 1 armed 10 year old could of done the same thing.