I think she just left them because in one of the episodes she had recorded a video for Miley, like she knew she was leaving.
Hold up--- I did find this:
Susan died at the age of 33 about the time Miley was 10 years old, likely from some terminal illness.
- Wikipieda the free encyclopedia
Yeah, I'm not entirely sure which was right. Maybe she left them and then died? The video seemed a lot like she knew she was going. It was the episode "He Could Be The One", I think.
