According to The Wrestling Observer Newsletter, the reason why Summer Rae was released from WWE was not necessarily because she did something wrong, but rather because WWE officials basically gave up on the original vision that they had in mind for her.
When she originally joined the roster, their long-term plan was for her to be a fairly major attraction, not so much as a wrestler, but more as a popular star on the program along the lines of Stacy Keibler or Torrie Wilson back in the early 2000's. But as time went on, WWE officials didn't really see her in that same light anymore.
Another thing that hurt her career was the storyline between Summer, Rusev, Lana, and Dolph Ziggler back in 2015. Again, although Summer didn't really do anything wrong, the storyline was considered to be a blunder. After that, WWE officials couldn't really think of anything else for Summer.