That's a very neat video. Here's what I think is happening:
It has nothing to do with the rice. Basically the frequency is creating a standing wave on the 2-d surface which has nodes in the shape given so the rice will bounce around erratically unless it settles on a point on the surface that isn't vibrating and then it'll just stay there. The one thing that confuses me is that the sound you hear in the video seems to be only one pitch and far too high. I'd think there's actually a lot more complicated signal going on in the low frequencies (below human hearing) to create this effect.