I can't say what is causing it but maybe just hormones or something, not sure of her age, but maybe something is triggering her to have more of a sex drive or having "wet dreams" have you asked her about her dreams? I know sometimes ive had some steamy dreams and sometimes you can be dreaming about maybe you being very pleasured by something, and it feels real cause shes pleasuring herself in real life, hopefully this isnt too confusing sounding, I don't think its a big problem, im sure if shes honestly not purposely doing it, it will pass. But you saying you guys have a great sex life, I wouldnt worry, but definatly ask her about her dreams, or maybe shes fantazing things through her mind before she goes to sleep (maybe even thinking about the sex you guys had earlier or something) best of luck : )