Why does editor reject when reviewers recommend acceptance?
First, a note: the editor (or editors, or editorial committee) is solely responsible for the decision to publish or reject a submitted manuscript. Reviewers are often (not always) called in and their reports provide help to the editor in evaluating the manuscript. But, at the end of the day, it is the editor who makes the call, so the situation you describe is possible and not necessarily unethical.

It is, however, very unusual, both for the editor not to follow the unanimous recommendation of the reviewers, and even more so not to explain their rejection any further. You can definitely (and, in my opinion, should) contact the editor to:


express your surprise at the rejection, given the contents of the reviews;
ask whether it may have been an administrative error (with the nice streamlined web-based editor interface, a simple misplaced click might have lead to the current situation);
if the decision is deliberate, ask the editor if he may expand on the reason behind the rejection.


Of course, be professional and polite.



If after contacting the editor you are not satisfied with the answer, the only way forward is to appeal the decision to the editor-in-chief or the full editorial board. Details on how to do so should be found on the journal's website. This should not be done lightly, but if you do not get a decent reply from the editor, it seems warranted by the facts of your case. You may, however, want to consider sending the manuscript to another journal, as it will take much less effort than the appeal…