If I were an employer, I would ask myself, ..why would I employ someone who has already left? then, wants, for whatever reason, to come back??
They are obviously capable of leaving again, so why should I invest time and money, when there are so many other candidates out there who might, be a little more loyal to my cause.
OK, they may have left to better themselves elesewhere, at someonelses expense, but, am I yet another short term stepping stone?
Continuity is what it's all about for employers. They 'might' find it from a new recruit, but what chances from someone who has already shown that they will leave at the drop of a hat?
Having said all that there may well be some good reasons, but, they would have to be bloody good!!