When the boss suddenly lost his temper, nobody could figure out why.
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From Li Ruzhen's Qing-dynasty novel 《镜花缘》 (Flowers in the Mirror), chapter 40: 二人摸不着头脑，又不敢违拗 - two characters can't make sense of an order but don't dare disobey. The literal image: grasping for an animal's head and finding nothing.
grammar
The structure 摸 + 不 + 着 is a potential complement: 'unable to touch/reach.' 摸得着 would be the positive form ('able to grasp it'). This is a colloquial idiom; 头脑 (head/brain) stands for 'the point of the matter.'
