Perhaps a little bit of speculation here, but I think it might be the pressure from the tight line that stimulates the fight. When the fish can not go where it wants to on its own whim, it starts to fight to get there. I say this because of the sequence I experience while nymphing, since I'm a lousy hook setter. If I see the strike indicator go under, it isn't until I tighten the line (since the hook already seems to have set itself, indicated by the strike indicator going under) that the fish really takes off. I realize that the strike indicator could technically go under without the hook being set, but the small size flies I am fishing, and the way the fish is hooked when I land it, suggest that the hook often is set when the strike indicator dives.