如何解释这里的 plugged？

有一读者问：高等教育出版社出版的 Modern English 中有一句：

Young people walk about plugged into their stereo walkmans, which is their choice.

此句中 about 之后为什么用过去分词 plugged?

plugged into their stereo walkmans 是一伴随状语，它与 about 没有直接关系， walk about 是一常用词组。