why do twins self distruced when they go close to someone

Twins self destruct due to its splash damage.  If the distance between you and your target between 1 to 4 metres,  then twins will cause self-damage due to it's splash damage hence some times result in a self-destruct. So protection is naturally advisory. 

If you want you can remove the splash damage from twins by purchasing "Stable plasma" alteration of twins. Read more about alterations here. https://en.tankiwiki.com/Alterations