The Nike break up must not have been pretty. Kara talked about going elsewhere when her contract was up. Nike had a right of first refusal with Kara and had to release her in order for her to be able to go to competitor. Nike must not have been willing to match whatever she was being offered at the time. Just imagine if Nike is paying you seven figures for years and then low balls you when your contract is up. Nike dropped Meb for being too old. The cold hearted capitalist at Nike were probably not very touchy feelie when negotiating with Kara.
Kara then signs up with Oiselle. Her arrangement with them has been described as "unconventional". I suspect that they gave her stock (Sally describes her as a "business partner" in the press release announcing Kara coming to Oiselle). That is all start-ups have. So, if Oiselle can get even a small piece of Nike's market share, Kara could really rake it in if she got stock for her endorsement deal.
I have also heard that Salazar could be merciless with Kara in training. Let's just say that he was not the kind of coach to say: "you look awesome out there!!!" Salazar would motivate by trying to get his athletes pissed off at him. That kind of coaching can get results, but it can also foster resentment down the road when you realize that you probably did not have to be miserable in practice in order to improve. Jerry Schumacher is very much the anti-Salazar. Kara may have come to find Salazar's coaching to be more abusive that constructive when she left and started chasing Shalane around with Schumacher's group.
But, I think more than anything, she just did not want to bite the Nike hand that fed her. If she went public in 2011 with allegations, Nike might dump her if the allegations turned out to be untrue. Now that she has Sketchers and Oiselle, she does not have to worry about losing her paycheck.
It is a valid point that she has made a big about face.