Why would the human population shrink 30%?

That depends on whether or not economies will collapse across the globe.

If they do, or the migration pressure throws countries into war, then all bets are off. You could plausibly raise that estimate to 60-70% (entire urban population + some margin). Cities aren't self-sufficient wrt. food. Many countries aren't either. So if, for some reason, national transport and power infrastructure goes down for good, you can scratch city dwellers off the map.

It’s only taken the last 25 years to add 30% to the human population, and the rapid increases of the last 50 years owe a lot to reduced infant mortality and people living longer.

So a rapid decline might come from a reversal of those trends.

It is just an example.

Some people mention an extinction level event.

I mention a number that is horribly catastrophic, just to show that even that huge number is very far from an extinction level event.