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.
Whilst the latter event is possible on a local level it is a very unlikely occurrence globally hence we'll move on to the collapse of the global economy and the premise is conditional on the collapse not caused by any other extinction level event, i.e. this is a singular event that occurs without, for instance, an asteroid strike or pandemic.
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, event global economic collapse is unlikely to push humanity to an extinction level event.