What could've made an electrical wire in my attic burn?

Found a wire in my attic right by the hatch with about 2" of it burnt. I didn't see any bare wire exposed, but pretty close. It's not at a connection. There is a little bit of insulation singed around it.

Maybe I nicked it when I climbed up there a few months ago? Maybe a squirrel or something was gnawing on it? I didn't see or smell anything dead.

Or could it be it's just wired wrong?

I'm pretty worried because it's connected to my smoke detectors that were wired last fall. I have 6 of them and they are all connected to their own separate breaker. So if it starts a fire and shorts them out I would never know.