I'm not sure to what level of detail you'd like the information so I'm just going to go for it.
Individual neurons form circuits, which form networks and then super-networks and so-on. Electrical activity, when viewed on a mass scale like this at the highest organization of neurons, produces electromagnetic waves which can be recorded by electrodes placed on the scalp and defined according to their amplitude, periodicity, frequency etc.
In the waking state the brain exhibits specific waveforms in specific areas of the brain associated with various tasks according to the electrical firing of neurons on a mass scale. This is also true for the sleeping state, but the waveforms between these two states are fundamentally different from each other. In between you have a transition state, defined by the theta frequency waves (3-8 Hz). Before sleep, your brain transitions from alpha waves (8-12 Hz) to theta. Alpha is associated with concentration and introspection, while theta is associated with memory and subconscious access. You are technically awake during this transition from alpha to theta, as in you have the ability to consciously analyze your own thoughts and can control your body's functioning practically as normal. This is when hypnagogic hallucinations occur, during the transition state. (http://www.sleepeducation.org/sleep-disorders-by-category/parasomnias/sleep-hallucinations/overview-facts
You say you notice your hallucinations most prominently right before bed, ("as soon as my head hits the pillow and my mind isnt occupied"). When your mind is occupied, as it is during waking hours for the most part, it exhibits more beta (12-38 Hz) and gamma (38-42 Hz) frequencies. Since hypnagogic hallucinations aren't associated with these frequencies, it makes sense why you would not experience them during the day when you are busy.
"The transition from waking to "floating" (or sleep) is not simultaneous in all parts of the brain. The alpha waves may be suppressed and the delta waves appear in one part while the alpha rhythm continues normally in another region. On such occasions the subject may report that he has been fully
Changes in Human Brain Potentials During the Onset of Sleep) also take a look at (http://onlinelibrary.wiley.com/doi/10.1111/j.1440-1819.1998.tb00998.x/pdf
You also mention meditation as a previous source of these hallucinations. Meditation states have been found to exhibit theta and alpha frequencies, just as the phase transition does.
"Although the neuroelectric correlates of med-
itative altered consciousness states are not yet firmly established,
the primary findings have implicated increases in theta and alpha
band power and decreases in overall frequency." (http://sccn.ucsd.edu/~arno/mypapers/Cahn06Review.pdf