Read the passage given below and choose the option that best fits the question that follows:
He drooped off to sleep. The cigarette slipped out of his mouth and burnt a great black hole in his only shirt. The smell of the burn awoke him, and he got up, cursing under his breath, and fumbled in the dark for a needle in order to sew up the hole. Otherwise, his wife would see it in the morning and would nag away at him for a couple of hours. But he could not find a needle. He fell asleep again.
The cigarette fell out of the man's mouth because _____.