An editor can ask for coherence. He can also throw out the chapter if there are omissions that he thinks are critical. He can, in that vein, request that references be included.
But the responsibility on the article lies with the authors, and so the editor can never modify the content of the article without their approval; he must ask their permission. This falls out side of his "jurisdiction" (exceptions can exist in commercial publication/scriptwriting where the rights on the material may not fully lie with the concrete author).
Imagine the extreme case of the editor quoting some crank papers under the author's name.
In the concrete case, OP should consider:
- whether the citations are, content-wise, appropriate and acceptable or not;
- if they are acceptable, whether OP wishes to place the matter of the unauthorised addition on the table;
- if OP decides to put them on the table, whether they would withdraw the article if the editor's justification is unsatisfactory.
Depending on the responses, OP may consider to
- withdraw the article; or
- insist on the references being removed; or
- accept the references and not work again with this editor in the future; or
- accept the references as a welcome addition to the paper.