My 8 month old has started doing this all the time especially when he’s in the car seat or has a hat on. He’s teething only has one tooth I put it down to that. And his hair is getting long and noticed it and is pulling it also so maybe thought his head is ichy. I e heard it before also might be a learning his movements thing but I wouldn’t worry xx
My almost one year old does this and has been for a while. We think she just likes the feeling of it and she mostly does it when shes got teeth coming or is playing with her hair. Does it when she’s crawling sometimes and we call it her sassy wiggle.
Maybe they've just learnt how to do it so like doing it?