I would suggest that this is because, during meditation practices, there is a decrease in what C.G. Jung called the "threshold intensity" of the conscious mind - that energy force that keeps unconscious material held at bay. Once the energy tension of the conscious mind is allowed to relax, the threshold intensity becomes less active, allowing unconscious material to be released into awareness.
When this happens, a hypnogogic feeling of being in that twilight region between sleeping and waking can occur. Time begins to alter dramatically, often seeming to slow down, and unconscious thoughts, images, and other events can be seen more clearly. These events often take on the same quality as dreams.
It is also entirely possible that we simply fall asleep for short periods of time while in sitting meditation. At these times, of course, the threshold intensity of the conscious mind decreases naturally, unconscious material arises, and the awareness of the unconscious material takes on the quality of pure dreaming.
Dream: A poem by Sukasah Syadan:
last night I dreamed
that I dreamed that I awoke
a sleepless man writing about what
I dreamed last night