The reason for this movement is that the shoulder is a ball and socket joint but the socket is shallow so the ball moves with great freedom. This essentially means that your shoulder is quite unstable. That is why your shoulder muscles are just so vital to a normally functioning shoulder.
In most cases, if you are suffering shoulder problems it is because your muscles are simply not strong enough or they are uncoordinated.
Luckily, most of these dysfunctions can be normalised after a quality assessment and the provision of injury-specific exercises from your West Active Physiotherapist.