“But why don't you scream now?” Alice asked，holding her hands ready to put over her ears again．

holding her hands ready to put over her ears again为现在分词短语作asked的伴随状语。

hold在此意为“put a part of your body into a particular position"

ready为形容词，其后的不定式修饰ready。

ready to put over her ears again为形容词短语，作hold的宾语补足语。

译文:

"你现在为什么不尖叫？”爱丽丝问道，同时她双手放在耳边，随时准备再次捂住耳朵。