[1]http://blackfridaydeathcount.com/ Crowd control is difficult,
   whatever the venue.
   [2]http://matadornetwork.com/nights/10-deadliest-concert-disasters-of-the-last-50-years/
   [3]https://en.wikipedia.org/wiki/List_of_violent_spectator_incidents_in_sports
     The authorities who run the venue try to anticipate problems,
   as one would with any large event and it was a failure in crowd
   control. They are investigating and will attempt measure to
   prevent it happening in the future.

References

   Visible links
   1. http://blackfridaydeathcount.com/
   2. http://matadornetwork.com/nights/10-deadliest-concert-disasters-of-the-last-50-years/
   3. https://en.wikipedia.org/wiki/List_of_violent_spectator_incidents_in_sports