Why do I just randomly shut down?

There could be a number of reasons someone shuts downs, though one of the biggest reasons is when our bodies perceive there is a threat to our felt sense of safety. For example, it may be one way we attempt to draw less attention to ourselves, one way we avoid the pain that can come when attempting to connect with others, or in an effort to avoid becoming overwhelmed by painful emotions. Shutting down is a type of trauma response one can develop as a child (or in other contexts) to help cope and survive a situation where they may have felt powerless or helpless...a situation when they had little ability to act or move or change their present circumstances. It's how the nervous system is designed to respond to issues of safety, whether it be physical, emotional, psychological, spiritual, etc. In other words, it's not something we choose. These reactions just take over to protect us.

Nevertheless, because we store implicit memories in our bodies we can also carry these trauma responses into adulthood with us and we can find ourselves stuck in particular ways of being even when the threat is no longer there, which may be what you are referring to now. Becoming more aware of our bodies and how our responses are linked to our life experiences/personal story can be a helpful place to start. Engaging in mindfulness techniques is also one way to help your body come back "online" when you start to notice yourself shutting down. Sometimes we refer to this as moving towards an "active state." This involves finding ways to slowly introduce an action of some sort and bringing mindful awareness to it. Engaging in anything that stimulates the senses can help with this. Some ideas include smelling essential oils, slowing moving one part of your body, squeezing a stress ball (maybe even making your own and using rice in it), chewing crunchy food, moving to music, painting with your fingers, water-play, etc. The idea is to find whatever works for you in order to increase present moment awareness and bring the body back into a more regulated place and beyond its present inaction.