Change in water temperature is the most likely culprit. However Brown Trout and smallies can coexist with each other.
Other things that can affect it are weather events during spawning season. Here in Wyoming, on the Gray Reef the Rainbows are doing very well thanks to flushing flows each spring, while the brown’s are doing terrible, as they have stopped flushing flows in the fall that allow brown trout prime spawning areas.
Though this year, might be an exception since flows have been kept high all summer long, thus less sediment is likely to accumulate on the gravel beds.
I have never caught a lot of smallmouth, just enough to make me really want to get a nice one on a flyrod. There is an island separating the main shipping channel to the lock at Wilson Dam and the rest of the river on the Tennessee River in Alabama where I dream of drifting a wooly booger along the rock bank. I was told for years the TVA diver and fishery people said there was very likely another world record in the vicinty. I would just like to catch one of her grand children.
the smallies were caught out in open areas where the water temps were up constantly.
I fished the wooded area this morning and caught 23 browns with no smallies to report.
If they can coexist it is just fine with me!
I target smallies, LM, and stripers 99% of the time and have found that smallmouth have an incredible range of water temp tolerance. I catch them in water that has trout and I catch them in water that is really hot. It may not be water temp but maybe a localized increase in a favorite forage in that particular stretch of river.