1.7 million people in South Australia were without any power last night: https://www.theguardian.com/australia-news/2016/sep/28/south-australia-braces-for-storm-that-could-be-most-severe-in-50-years. While I'm no electrical engineer or network engineer, here is a very simplistic version of what happened and why.
A national electrical grid like ours is a bit like Goldilocks. It likes to have just the right amount of electricity flowing through it. Not too much, not too little. And that ‘right’ amount keeps changing of course, every minute.
Yesterday afternoon it got too much (in a matter is seconds some transmission lines went down in a tornado, so the supply suddenly exceeded the demand) so a safety mechanism kicked in and shut it down. When you are sending all that power down the line (and from various renewable and non renewable sources - it actually doesn't matter where those electrons come from) and then there isn't a use for it - well we have a problem.
Within a few minutes the demand for power last night went from 1624 MW to 26MW as the Australian Energy Market Operator determined (or most likely it automatically happened) that it needed to shut off a large section of the grid in that state.
Fast forward six months and it's 44 degrees with a million air conditioners on. If (or when) the grid doesn't have enough power in that region- it will again shut down.
If it didn’t shut down yesterday, then electricity flowing through the transmissions lines doesn’t just disappear, and things will blow up (ok I'm being dramatic but the system and electricity doesn't like changes to loads and frequency). See https://twitter.com/KetanJ0 for some good analysis and commentary on the issue.
What's the answer? Well it isn't an easy fix. More decentralised networks that can connect into the grid when needed would help. And less demand for electricity through efficiency measures helps too. And maybe a greater appreciation that extreme events will happen and we can’t build a system that is 100% reliable!