In reply to Johnny_Grunwald:
> The video link supplied explains it perfectly, so thanks for that.
As Toby has explained, the video demonstrates one way in which failure could happen.
However, as far as I understand, it's not believed to be what happened in the sad incident you refer to - where it seems the quickdraws were simply assembled incorrectly, probably by someone who couldn't reasonably have been expected to know what was correct, and what was incorrect.
I realise that involves some degree of comment on the incident, but I can't see how it's possible to answer your question without.