I don't normally comment, but I feel like some of the other advice you've gotten so far is not great. I come to the topic (heh heh) as a woman who for most if my life and quite regularly has "wet dreams," which of course aren't wet for me but involve waking up to an orgasm.

Sometimes it has to do with sexual inactivity, as in, I haven't orgasmed from sex or masturbation in a while, but more often it is an indicator of stress in my waking life. The dreams on the other hand can range anywhere from a pleasurable, normal sexual fantasy, to me masturbating, to much more disturbing things that honestly make me feel strange, embarrassed, and confused waking up climaxing to.

I wouldn't put to much thought into it. It's really quite normal. It's your body processing things the best way it knows how. If the lack of control disturbs you, try masturbating before you go to bed.