I can see people getting burned out, especially if you're trying to turn out a lot of ponies for sale or commissions (nothing wrong with those things, of course- I just find I get tired of them faster than personal projects). Or just losing interest/moving on to other interests or hobbies. I've done it with lots of things. I made cards for a while, made terrariums, made sock monsters, one after the other. I've been doing ponies for longer than any of those, and I think I've finally found the perfect creative outlet for me, but I understand how it could happen!
People's lives change, too. Ponies aren't the cheapest hobby, unless you're selling (I'm always happy when my hobby can pay for itself), and financial situations change. New family members, living in a new place, a job that takes more of your time... I can see any of those leading to setting ponies aside for a while, if not forever.