because it whispers in the heart. so when allah is mentioned it withdraws. it is called sweeping because it hides as gazelles sweep meaning they enter a sweeping place.
لأنه يوسوس فى القلب. فاذا ذكر الله. خنس. وسميت كنسا بالاستتار كما تكنس الظباء أى تدخل فى الكنس ن.