It doesn't look like the article as a whole is downloadable without an account, but from what is accessible:

The Commission identifies two threats as having extinction-potential:

• The malicious use of artificial intelligence (AI)
• Nuclear conflict

I think that it's reasonably likely that by 2100, we will have at least two other potential candidates (if we aren't there already):

• Engineered, self-replicating biological organisms that create pandemics or other situations problematic to human life (e.g. if we produced something like the Great Oxidation Event on a short timeframe). We are increasingly able to craft and modify life, and I would expect that we will explore all sorts of possibilities.
• Self-replicating nanomachines that do similar; something like the gray goo scenario.

In all cases other than nuclear conflict, I think that rapid replication that spirals out of control is the common factor that could render a risk existential.

I also do not think that the most-likely existential risk from AI comes from intentional, malicious use—the article is specifically identifying malicious use—but from an error coupled with a system that can expand massively and spin out of control. Something more like the paperclip maximizer scenario.

Nice. Should be able to get my life over and done with before all that.