Good question wwh! The phrase is in common use in Oz and, like so many others, I guess it is so common that its etymology isn't questioned.

I'd always assumed that it was like the domino effect - if one event happens then another must...and another...and another etc. The etymology would thus be a reference to the initial action knocking over the next item in line.

Thinking further, I suspect this is purely fancy and that there's a far more robust origin!

Looking forward to the outcome of this one.

stales