Hi George,
I won’t be able to answer why it is like that. I can only answer how it works.
Firstly, your sentence is in imperative mode, I mean that it is similar to an order.
Secondly, it is a reflexive verb such as:
S’arrêter
S’habiller
Se taire
Se cacher
Se rendre
…
In these conditions, sentences are built like the following positive-negative forms:
Pos. : [Verb] [pronoun] !
Neg. : [Ne] [pronoun] [verb] [pas] !
Pos. : arrêtez-vous !
Neg. : Ne vous arrêtez pas !
Pos. : Habillez-vous !
Neg. : Ne vous habillez pas !
Pos. : Taisez-vous !
Neg. : Ne vous taisez pas !
Pos. : Cachez-vous !
Neg. : Ne vous cachez pas !
Pos. : Rendez-vous !
Neg. : Ne vous rendez pas
Remark:
Imperative mode is used for 2nd pers. sing / 1st pers. plur / 2nd pers. plur.
I have written only examples for the 2nd pers. plur. However, it works for other persons as well.