I think it's just a new way of responding to stress, or a situation he dislikes. He did it last night in bed, I'm pretty sure he didn't want to go to bed yet. Then this morning he did it when I put him on the bus (started getting angry and biting the seatbelt) which is new as he generally loves to be in cars.
Thanks. He's got a strict routine at night. It has generally only been happening in bed. He's got that bleary look where he is clearly fighting sleep. Or on occasion he wakes in the middle of the night, and he'll do it when he clearly doesn't want to go back to sleep. I was chalking it up to that.
But when I saw it this morning on the bus i became preoccupied. I was worried actually it was something on the bus as he loves school and is happy as a clam all day. I just had a long convo with one of his teachers. She said she was going to talk to the driver today, but as this is the first time it's happened just be on the lookout if it happens again.