I am reminded of some experiments I did around 40-45 years ago, filling relatively small  (1 foot diameter) balloons with stoichiometric  https://en.wikipedia.org/wiki/Stoichiometry     mixtures of hydrogen and oxygen (or another fuel gas and oxygen, example propane), if ignited they produce remarkably loud-sounding, and yet low-pitched "booms":  Sounded like a huge explosion done a long distance away.  Yet, there was no possibility of shrapnel, other than a thin rubber membrane.    This could easily scare people in a crowded venue, causing panic and crushing crowds.

Perhaps this explains what is being reported as happening in the Manchester arena.

              Jim Bell