Sorry! I don't know what to say about this. I wish I could help you but I think I find some sort of relation between talking in sleep and falling.

He is talking in sleep means he is probably dreaming something or imagining what happened throughout the day. So when he is talking to people he is responding with some sort of body movement. When he reaches the state of higher emotions like being happy or angry, he moves a little more than usual. The normal movement combined with this extra movement slowly takes him away from the bed bit by bit (wrong words?) and eventually he falls down!

May be you can put something beside the bed that will prevent him from falling down but I am really not sure if I am making sense. Am I?

if you concerned he may hurt himself by falling out of bed one talk to him and let him be aware.... I used to do it too. and two put the bed against the wall