Why does sleeping position in the bed cause sids?

Not known. The exact cause of sids is not known; however, there are risks now known. One of these is body position in bed. The idea behind back sleeping is that neck muscles and trunk muscles are not entirely formed so airway compromise can be more likely to occur and unlike in grown child or adult, babies cannot turn themselves over.
Abnormal Breathing. It is thought that some babies have an abnormal breathing " recovery" due to a potential problem in the brainstem. When they are lying on their bellies, it is harder for them to " recover" and bring in air with their face in the bed. This is why back sleeping creates a much lower risk of sids within populations.