From what I've been able to piece together, Ayman-al-Zawahiri, SPECTRE's* - ahem - Al-Qaeda's 'Number Two' man went on Arab TV and mentioned that they were planning a major terrorist attack around the holidays in the U.S. of A. There may be other indicators we may not be aware of that caused the powers that be to ratchet up the threat condition to Orange.
Al-Zawahiri is considered just as dangerous as Osama, and some say he's even worse than his boss. Unusual to see him instead of the main man delivering a threat, and that's something that's keeping our defenses up on guard.
* SPECTRE - The
SPecial
Executive for
Counterintelligence,
Terrorism,
Revenge and
Extortion. Had to insert a sly James Bond reference, but al-Qaeda operates very much like the fictional nemesis of 007.