My son still does this occasionally (he's nearly 6). He is a very sensory child and we have been address this (described here) for the past year. If your grandson is craving sensory input, they it is not a bad thing. Are there any other signs of him sensory seeking? Such as:
- chewing toys, pencils, etc...
- bumping into things and people
- rolling on the floor if stressed or overloaded
If he is sensory seeking, you need to work with him. He is already self-medicating, you (and the parents) need to support that process and help him get better.
Another possible cause is that the child may lack core body strength. Many children these days do not have a strong core so it is hard for them to sit straight. This can be seen, for example, in the way they write/draw. A weak child may hold their breath while drawing to help keep their body rigid.
Or it could be something else but keep in mind that children rarely do things for no reason.