My thoughts:

Crappie could have been introduced with the original stockers, and can be mature at 1 year of age and 3" in length.

Crappie could have been stocked by a "friend" or neighbor.

Crappie could have arrived from upstream or downstream.

If you are now catching intermediate size crappie you need to manage to keep crappie recruitment low, and start catching and removing the existing population. In theory, you should stock heavy with the largest shiners you can find. They may help control crappie fry. Do this quick because crappie probably already spawned and fry will be actively growing. Then the shiners will spawn soon and feed your predators. By stocking heavy, I mean 10-20 lbs per acre. Then if you think this works, after observation over time, you should stock heavy with shiners every February/March.

Also you can stock some HSBs at a rate around 25/acre to help keep crappie numbers down.

It's ALL about the fish!