This is most often noticed in tincs... but I've seen a number of other darts (pretty much the whole tinc group except leucs) do it as well. They basically start it up when you feed them or when they think they are getting fed. This attracts other frogs because its movement and basically a sign that a frog found food.
As for eating the platy fry... well, its moving, if its right at the surface they probibly think its just a fly or insect on the water. Frogs don't make a habit of eating fish, but I would imagine mistaken identity means they might occassionally grab one but don't count on them controlling your fish population :)