They often want to spend more time with family. They have hobbies that they didn't have enough time to devote to while working. And, for a lot of later career people in tech, they weren't doing a lot of coding anyway--and it's not like they can continue to meet with customers or set company technology direction part-time over the long term although a few very senior folks may stay around as an advisor for a time.
How many people? Any commonalities surrounding their choice?