Why would the editor in chief add a new reviewer after the first round of revision?

While this is rare, it's not necessarily bad, and probably reflects more on the associate editor or the reviewers than you.

One of the jobs of an editor is to ensure that reviewers are sufficiently well qualified, objective, and thorough in their evaluation of a manuscript. Likewise, an editor-in-chief needs to ensure the handling editor is doing their job well as well.

It's hard to say without more details, but my guess on the most likely cause of this situation is that there was some issue with the original set of reviewers, such as:

• Too many were your recommended reviewers
• There was a critical missing perspective
• One or more of the reviewers turned in a low-quality review (which the editor might have forced them to improve before it got to you)
• The journal usually requires more than three reviews

This could be caused by bad choices by the handling editor or by having more than three reviewers originally assigned but some failing to return reviews.

Whatever the case may be, I would recommend not worrying about it too much. You can inquire with the editor if you wish, but don't be surprised if you don't get a particularly informative response (especially if the reason is a mistake they might feel embarrassed by).

Bottom line: it's not so strange, and your paper is probably still in good shape, though its fate is never certain until accepted.