With me I think it's just being forced to lay on my back that does it. It doesn't matter what I'm listening to, either; eventually, if I'm there long enough, I'll fall asleep. The problem is that I can't stay asleep, since I end up waking up after about an hour feeling like I'm stuck in a casket. Not being able to move my head makes me feel claustrophobic, and that wakes me up.
This actually upsets me because I often like using music to try to drift off to sleep, and it always works. But it's only temporary. One of the reasons I'm on an epic quest to find an IEM I like is because maybe if I can move my head I'll be able to stay asleep once the music lulls me off.