Popped out? Sprung out? I can't find the correct word.
She tucked a strand of her chin-length hair behind her ear, only to __ again.
She tucked a lock of her chin-length hair behind her ear, only for it to flop again.
Oxford Living Dictionaries gives this definition of the verb
[with adverbial] Fall, move, or hang in a loose and ungainly way.
- ‘his blond hair flopped over his eyes’
- ‘Some of his dark hair flopped onto his forehead and I almost reached out to tuck it back into place.’
Depending on how poetic you want to be, you could say:
She tucked a strand of her chin-length hair behind her ear, only to have it emerge again.
(Or you could use @BennyLewis' phrasing and go with ...only for it to emerge again.)
Less poetic would be:
...only for it to come loose again.
Any which way, hair is a pain. (says the guy who hasn't had a haircut in 7 or 8 years.)
You do your hair (into a hairdo) by arranging it. When it becomes disarranged, we can say it comes (or becomes) undone.
She tucked a strand of her chin-length hair behind her ear, only to come undone again.
Do can mean to set or style the hair or makeup. (The Free Dictionary #6). A couple of years ago, the undone look became a fashionable way to style one's hair and dress.