How do wet dreams (ejaculation while sleeping) happen without no physical stimulation? Is there any theory on why they happen?

Nocturnal emmision. Wet dreams occurs mostly in teen males, it is happened when the seminal fluid accumulates in the seminal vesicles and be overfilled so the nerve endings in the seminal vesicles wall stimulates the parasympathetic nerves and leads to ejaculation during sleep, it is less occuring in married males who have regular sexual relations.
Brain mediated. Ejaculation and erections can occur without any physical stimuli and can be mediated by your brain alone. It is normal.