simples. You are more comfortable on your back. In sleep as in wakefulness, we will move around to the most comfortable position and once found will tend to stay in it.
I have this really annoying habit of sleeping on my side. But I wake up on my back with my arms above my head with really bad pins and needles in my shoulders. I have even concidered tying my arms to my waistband so I don't do this. Surely that is not my most comfortable position.