...maturity level, injuries, breeding season, and amount of growth (related to amount fed).
A snake which is still in a major growth phase will shed much more often than one which is 'full grown'. (Snakes never quite stop growing, but they will slow down sharply as adults.)
A snake which is injured or sometimes ill will shed more often as the snake tries to heal the injury.
Breeding season brings about timed sheds in adults. Females will shed before becoming receptive, before laying, and after laying. Males will typically shed after coming out of brumation as well.
Amount of growth -- this is somewhat related to the first response. A snake which is growing alot will shed more often than one which isn't growing much. A young snake fed very little (like it sounds yours was previously) will not grow fast and will not shed as often. Once you start feeding it more often, it will grow faster and shed more often.
I've also noticed that if you feed snakes after a period (atleast a month) of non-feeding, they will often go into shed. I've mostly seen this around brumation time... the only difference is, with this shed nobody is ready to breed yet.
-Kat
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"You keep WHAT in your freezer?"
"Mice. And rats. If that bothers you, I can call them 'cows' instead."