You're basically right - the sending machine has identified itself as "rooter", and if that was my home network I would work to identify that machine, since I would feel responsible. If it's an open wireless network at that guy's house, a neighbour could be piggybacking on it, which is why wireless security is a Good Thing.
The name "rooter" is a bit "script kiddie", someone aspiring to be a hot-shot hacker. Or a Strayelian?
