I've read theories regarding this. One theory is that when the pressure is high there is more pressure on the fish's stomach which makes it get the sensation of being full and hence not apt to feed. During falling pressure or low pressure obviously the pressure is lower and the fish's stomach would feel empty making the fish more apt to feed out of hunger. Not sure if its scientific or not, but it is a theory. There is no question the barometer affects the way fish feed, why exactly this is I don't think anyone really knows.