He finds that lawyers working with serial plaintiffs will file a bundle of “drive-by” lawsuits at once, slamming businesses with vaguely-worded grievances that leave many companies wondering what they have done wrong.

1. 现在分词短语slamming businesses with vaguely-worded grievances that leave many companies wondering what they have done wrong的意思是：

“用措辞含糊的不满来抨击企业，使得许多公司不知道自己做错了什么。”

2. slamming的逻辑主语不是He，而是lawyers。

3. that leave many companies wondering what they have done wrong，定语从句，修饰vaguely-worded grievances：（这些措辞含糊的不满）使得许多公司不知道自己做错了什么。