They don't know yet. Hubble was down for maintenance, but they quickly pointed a camera at Saturn to try to get some idea what's going on.
Maybe it's V'ger. Or the Enterprise screaming through, trying to beat it to Earth. (Sorry, thirty year old jokes are the best I can do. Still haven't seen Wall-E.)
Boy, the solar system is falling apart lately. First that scar on Jupiter's butt, now this.
And never mind that Chairface business...