In my own (non-technical) case, I had a great 15-year run as a top-of-the-heap IC -- and then my outfit got bought by people whose business model had no use for what I did. Time to do something different, and with it being 2008, going independent felt a whole lot smarter than trying to find a new star-IC role in a scared industry.
Going independent was good for 7-8 years, but then my favorite partners at my favorite clients all got to the end of their roads. New faces; new visions, and the unappealing prospect of spending 3-4 years trying to revive a shrunken pool of business into something better.
A star-IC role opened up somewhere else, and it's been a great ride ever since.
It's the career equivalent of summers in Alaska; winters in the Caribbean. Sometimes the key to staying warm and happy is to be willing to move when the temperature changes.