After author response a new review appeared---where did it come from? How can I respond to this new review?

During the post-author-response phase, meta-reviewers and reviewers engage in substantial discussion. In some cases, a meta-reviewer may decide that they need another opinion. They are encouraged to solicit this additional opinion either from their own network, or from our pool of "expert reviewers" whose job it is to step in at the last minute and help make difficult decisions, especially when there is disagreement among the initial reviewers, or when some issues are not resolved by the author response (or, in some cases, when additional issues arise as part of the author response period). As a result, you may find that you have an additional review for your paper that you did not have before, solicited at the meta-reviewer's discretion

You may wish you could respond to this new review. This is simply not possible, given timing, etc. We recognize that this can be frustrating---for instance, if the new review is significantly more negative than the original reviews (also please remember that scores are not decisions---they are generally not well calibrated, and a meta-reviewer's job is to use scores as features, not as votes).

Please keep in mind that when new reviews are solicited, it's precisely because the meta-reviewer has engaged deeply on your paper (including in many cases reading and reviewing it themselves), and they are doing their best to ensure that all papers are treated fairly.