You can't say when it occurred. There is no means to differentiate a "cause" arc from an "effect" arc using laboratory analysis. It's been tried.

That said, almost all arcs are "effect" arcs. Unless it's a "series" arc, occurring at a connection point, it doesn't last anywhere long enough to ignite anything but gases and clouds of dust.

It may happen, but I have never seen an arc in the middle of a straight run of cable cause a fire unless the cable has been physically damaged. But I have seen fires cause many such arcs.