I haven't looked at the set up here, but I do know that Particle Flow lets you add things like deflectors, to bounce particles off of, so that could be done. Also, it allows you to spawn new particles when a particle hits something, so you could also set it up that way, so that an original rain drop particle can trigger a "mini explosion" of smaller particles, so it could be done that way too. Now, whether that's contained in the setup, or was what was used in this scene, I can't say :) But it would be possible to set it up so that the particle flow took care of all of that!