Is electrical Arc and overload the only potential fire causer for electrical wiring?
A fire is just uncontrolled heating, right? When a thing starts burning what's really happening is a chemical reaction that produces heat, which causes more reaction, producing more heat..
So the question becomes: how can electric power deliver enough heat in a small space to get a chemical reaction (fire) started?
Arcs are a great way. An arc produces a lot of heat without a lot of current. That's why regular over-current circuit breakers weren't good enough and we added arc fault detection too. But arc fault detectors are tuned for certain parameters. They aren't fool-proof; an arc that falls outside the detection criteria would not cause an AFCI to trip. Consider the arcing that occurs in a brushed motor and doesn't trip an AFCI. If conditions were right similar arcs could occur in the wiring, would not be interrupted, and could start a fire.
That said: one can still get plenty of heat for starting a fire with neither arcs nor over-current. Cooking fires happen all the time as a result of electric power delivering fire-starting heat without exceeding over-current limits.
It isn't hard to imagine that a deteriorated connection, or even one that just never was great to begin with, could produce some heating somewhere in the electrical system. Maybe it will get hot (resistive heating) but never arc even once. This could start a fire without being detected by AFCI or over-current. Torque specifications for connectors are part of the solution to this problem.