Not to make you paranoid, but I wouldn't put it beyond Linksys to have some sort of backdoor or callhome function in their router OS, to restore certain key settings. I don't think it's very likely, but certainly not impossible. If it shows up again...well, you'll have to draw your own conclusions.

BTW, you don't have UPnP enabled, do you? That would do it as well. And running P2P software on a UPnP enabled network...wow, that could be bad, if you had a shady or malware riddled client.