I was fishing last week, and took some time to observe the fish behavior, as the water was especially clear and the ecosystem was especially healthy.
Something I noticed almost immediately was that the largest fish were swimming near the shore. I've seen this before, but in this case there were no small fry near the banks. It was only the large adults. Out toward the center of the lake was where you were seeing smaller fish rise to food on the surface. In the case of this small mountain lake, the prime locations for what these trout need (food and cover, mostly) happened to be near the banks while the less prime locations were out in the middle.
As they say, big fish eat small fish, and if those small fish want an opportunity to cruise a better location, they will have to fight for it.
The battle for food and shelter/protection is not something we only see in humans fighting wars. The vast majority of animal species on this planet spend the entire length of their lives in a constant battle for food and shelter.