I have had that happen many times. Also with other species mixes in both fresh and saltwater.
The prey species are attracted to the smaller struggling hooked fish that is giving off all sorts of signals that say it might be an easy meal.
This is very difficult to imitate with a fly - maybe impossible. Hard and soft lures use rattle, wobbling action, semi random darting action, or other features that attempt to simulate an injured fish, but still don't quite match the real thing.
I have found that baitfish patterns that move very erratically sometimes out perform standard pattern. The erratically behaving patterns that I am most familiar with all have weight at the back of the hook or even trailing behind the hook. I haven't seen much in writing about such patterns other than discussions over the last 15 years on Dan Blantons website.
See http://www.danblanton.com/blog/blant...-instructions/
And
http://www.danblanton.com/blog/blant...bit-the-story/
For example
They still don't move as erratically has a hooked bluegill.