I think you're on to something here. This has kind of been my thinking all along. IF he did something to her, I think this could have been his motive.
Also, and this is absolutely just rumor and speculation, but apparently his FB page had some posts that caused some people to believe Katelyn was pregnant (they have since been deleted, but someone called him "papa," he made a comment that his best friends didn't know yet, etc.). If she did happen to be pregnant, I think a young woman in her position is bound to start doing some re-evaluation of her relationships, especially if she is considering keeping the baby.