TwoTwoLeft
o- - - - - - -l
While any number of factors could be possibilities here, I'd like to see what the investigation rules in or rules out first. Because in accident investigation, one thing I've learned over the decades is that things aren't always as obvious, or as straightforward, as they may seem at first glance. Any of the ideas put forth by persons here could have some, much, or even no percentage of factor to this accident. But just remember, nothing is 100%; and it can't always be said that things "have" to be a certain way or occur in a certain way; because when it comes to accidents, things don't necessarily "have" to be anything or "have" to occur in any particular way. Just saying, be careful with instant conclusions..
The video clearly shows WHAT happened.
You're right. WHY it happened, at this point, it's unfair to the performers involved and their families to speculate.