Gregg is pretty much spot-on correct. Late Fall through Winter months, Browns put the feed bag on but also are in spawning mode--thus very aggressive and territorial. Also, trout in general can and will be found in the shallows when the weather is cold. Colder it is, the shallower you will find fish. Combine the two and you'll find the browns in all the prime "trouty" spots. You might even find them in water less than a foot deep--if the prospect of food with good cover is there.
How do I know this is true? Working from some passing reference I once noted somewhere (something David James Duncan casually mentioned in one of his books?), I took the theory in hand and eventually found myself with some jumbo trout hens sputting eggs into my net. No, that wasn't exactly how I envisioned my plan working out, but, sans eggs, it was. Pattern used? A big, annoying, chartreuse woolly bugger. It was an "anti-attractor" pattern, and it worked. Strangely, the same pattern worked at Pass Lake when left hanging in the water while I was talking to someone. (Was that Ford Fenders at the time? Damn, it's been a while since I was last fishing seriously!)
Personally, I like stillwater fishing in Winter the best. That's when an Intermediate line really proves its worth in finding JT's. My problem has been me being shipped out during those months. For instance, I'm typing this while in Singapore. Not good.
The question of what changed was more one of reflection, versus literal. I fish Pass year round so it was not the season, I've have also fished the same methodology for awhile, so it was also not the technique. Browns are a nice change to the pace but I would never turn my nose up at a rainbow. What changed? Nothing, just a few more browns mixed in.
Pass does not always give up that easily, but at times now I can tell that it will before I ever hit the water. I was just talking to a friend today about when we first started fishing the lake in college and how poorly we did kicking around with our floating lines and unweighted flies. Oh how times have changed!