It's often a business that's past its heyday, and new management thinks they're smart enough to turn it around. And sometimes the problems are internal, and can be fixed, but rarely by changing the product itself. Other times, it's because the world has changed, and there's not as much need for the product.
Two that really pissed me off were Sorrel and Muck Boots. Both great products, which had loyal followings due to high quality and performance, and both have gone to total garbage in the last ten years. Muck Boots changed all of their fits and sizings about 6-8 years ago, and also the way they assemble their boots. They don't fit as well, and they don't last as long. Sorrel is even worse, very solid Canadian-made winter boots, now imported glued-together synthetic junk.