A later dip in brightness, which actually consisted of a series of dips in 2013, can be explained by a trailing swarm of asteroids — much like the Trojan asteroids that share an orbit with Jupiter.
What's so spectacular about WTF's brightness is that there is a single, smooth dipinbrightness followed 700 days later by irregular but large decreases that lasted for 100 days before the brightness returned back to normal levels.