I don't really think she was the one to walk away. In my opinion, it was a mere contract that simply ended and both parties parted amicably. Let's face it, if they truly wanted to re-sign Kate, they would have used her more during her contract and we would have seen more work by now. They had plenty of opportunities to use Kate but they didn't, the biggest being the opening of the first VS store in Russia (even in the announcement video they used Behati instead of her
).
I believe her limited contract was an agreement between her agency and VS. The Lions also manage Sara and Stella so my guess is her agent threw in Kate in the deal as well in order to raise her profile.