I'm assuming this a freak stage accident involving props, because this seems too bizarre a target.
Link.Edit: Sources saying multiple people dead, but a website called "Stuffz.NZ" doesn't seem credible to me. Maybe I'm just ignorant of Kiwi media. NBC has reported it on air and the term "multiple casualties" was used. Scary stuff.
Edit Edit: So Billboard is confirming multiple casualties, Fox is reporting it on air now. If this was a fireworks fiasco, Grande would have possibly been injured but she's confirmed to be unharmed.