I experience something similar when I don't sleep well the night before. The past few years have been like that. I've actually fallen asleep while soldering under a microscope with the soldering iron in my hand. Luckily, nothing has been burned that was of any importance. I've also fallen asleep while sitting at traffic lights. Again, luckily nothing bad has happened, but I understand the scary aspect of it. Keep in mind that this started about three years after I quit, so the two are unrelated for me.
What I experience is feeling a bit tired, but not really "drop off sleepy". Then I open my eyes and realize I was asleep, but have no idea for how long. I suspect it was usually for just a few seconds.
My problem, I believe, is that my bed needs to be elevated at the head, but for various reasons I have not been able to do that. With a flat bed, I wake up several times during the night, often not enough to be aware of it. My recent SO has commented that she has been aware of me coming out of sleep, then going back because I would stir or make a sound that woke her up, too. Together we were able to draw a loose and very unscientific correlation to her disturbed sleep (because of me) and my daily tendency toward "naps".
Perhaps your falling asleep is the result of several minor sleep disturbances due to nicotine deprivation. If that is the case, it should pass in a few weeks as your body adjusts to life without nicotine.
Are you using NRT or any medications?