I've been at "festival seating" shows where people crush toward the stage, and it's not pretty. The Pearl Jam Soldier Field concert in 1995 comes to mind. A couple friends and I were former offensive linemen, and we formed a 3-man "wedge" to push away from the stage and help some rather petite and freaking out girls through the crowd.
I also recall that security switched into lifesaver mode and pulled people who can't breathe/were getting crushed over the barrier in front of the stage.
Unfortunately, it doesn't always work like that — as PJ found out a few years later.
If people fall down in the midst of a crowd rushing the stage in a dark, hot, opening-seating area, it's easy to see how tragedies can happen. Sorry to hear about this show.