what happend status of the journal changed back to the under review from reviews completed?
After reviews are complete the editor has to make a decision about the paper. There are various possibilities between accept and reject including "seek more information". For some reason the reviews weren't sufficient for the editor to either accept or reject and so they sought more information - putting the paper back into review. 

It might have been simple like seeking a clarification from one of the existing reviewers or it might have been asking someone new (or several someones) to take a fresh look. 

It might even have been that the editor sent the paper originally to the wrong reviewers and so got back nothing useful. 

It is possible that they will respond to a request from you for the reason. Your situation isn't unique.