How are "global computer failures" an existential risk? Sure, it would suck, but it wouldn't be the end of the world.
Global trade depends on computers these days, and the human population depends on global trade to get food, medicine, building materials, technology parts, etc.; even if all humans would not be instantly killed by a global computer failure, it could stall or stop expansion.
And what are "physics threats"?
Vacuum metastability event, for instance?
I can see a global computer catastrophe rising to the level of civilization-ending, and 90-99% fatality rate, if I squint hard enough. I could see the fatality rate being even higher if it happens farther in the future. I'm having trouble seeing it as an existential risk, that literally kills enough people that there is no viable population remaining anywhere. Even in the case of computer catastrophe as malicious event, I'm having trouble envisioning an existential risk that doesn't also include one of the other options.
Are there papers that make the case for computer catastrophe as X-risk?
Due to my colleague, Anders Sandberg: