This is one of the least likely events that could cause an extinction level event.

Global System Collapse typically alludes to either a catastrophic collapse of the global financial system or the simultaneous collapse of virtually every government at the same time leading to widespread panic, starvation and death.

That being said, a global financial collapse could be caused by an isolated event in one of a few locations.

For instance, if a terrorist were to detonate a small nuclear device in the Port of Los Angeles or any other important port in the US or Western Europe, the resultant damage, whilst limited would have a catastrophic effect on the economy and that, historically, can become a downward spiral very quickly.

Again, though, even global economic collapse is unlikely to push humanity to an extinction level event.