Shutdown at this age can look like sulking, refusing to speak, going completely still, or physically removing themselves and not returning. Underneath is a nervous system that has hit its ceiling and chosen collapse over explosion. Kids this age often feel shame about shutdown, "I do not know why I do that," because it feels outside their control. It is outside their control. That is the point.
You do not have to explain anything right now. I am not upset. I am just here.