Draggability was added for TagMultiselectWidget from the initial implementation on. There has been a product idea behind adding draggability, first stated in T108490, but to my knowledge we're currently not supporting any functionality based on TagItems being dragged.
We should consider removing draggability until we have a clear product idea where to implement it. Or come up with latter.
Currently, draggability results in issues like T168762 or T183368.