Why does Mallinson return shortly after he left? (chapter 11)

Mallison returned because he was afraid.

His face was ashen and his whole body trembled. "I hadn't the nerve," he cried, half-sobbing. "That place where we were all roped—you remember? I got as far as that... I couldn't manage it. I've no head for heights, and in moonlight it looked fearful. Silly, isn't it?"