"Smart Network Switch" from the 4.3 Android update is a feature that supposedly detects whether your WiFi internet connection is stable. If/when it determines your WiFi connection is "unstable", or slow, it automatically switches to mobile data instead.
Some of the early iterations of this feature seem buggy, as they arbitrarily deem just about every WiFi network "unstable" after a while, and switch to mobile data without ever automatically switching back to WiFi (unless you manually reconnect to the WiFi network).
