My therapist suggests it is defensive or resistant. Defensively taking myself 'away' to a safer place is one idea. Resisting exploring what is causing the intense emotions that precede the 'nothingness' is another idea. I suppose there are more, depending on the person. And I think it could be more than one thing.
Sometimes this feeling of 'nothingness' feels like a relief. Sometimes it feels like depression. Sometimes it feels like the flip side of anxiety.
Much to explore.