Probably running out of ram on your device. I often have my games shut down randomly after playing for a while, when the scene is too resource intensive, or when a fight/H-scene gets particularly active in what's happening on screen. Because of this I've built a habit of saving a lot to.save progress. It's unfortunate android seems to have opted for "Let's shut down this app because it used up all the ram" instead of "This app used all the ram? Oh well! Guess I'll keep the bare minimum for myself, while the app stays running."