Why some autistic kids lay down on the floor anywhere?

My son is 6 years old, have down syndrome and autism, some times he lays down on the floor in his clroom or in the hospital floor or anywhere he feels like. Is this a behavior issue or have to do with sensory issues. His teacher usually put him in time out or remove him from the cl when this happen. I don't think he does in purpose but the teacher disagreed. How can I help the teacher to understand .