Yup. Only a couple times. Very scary. I agree, it's weird that in can happen on a bike.
This is the thing, it has never happend on any of my long Iron Butt rides. When riding those, I am way more attuned to my body. When I start getting drowsy, I stop. Sometime perhaps I should stop sooner, as I am fighting it, but I always stop.
The times where I actually dozed off have almost always been in daylight, when it's hot as fuke out, and after a good night's sleep. I don't get it. Other than I am a night person by nature.
Sometimes that big, heavy, drowsiness sets in for no apparent rhyme or reason. The trick is to recognize when that is happening, and take action immediately, as in stop!