見返る and 見返す are different from the usual transitive/intransitive pair. In these pairs, the transitive one is often about causing the intransitive one to happen to the object (e.g. 始める means basically “to cause 始まる”; 返す means “to cause 返る”). That’s not so here. 見返す does not mean “to cause 見返る”. In this case, 見返る is only intransitive because no object is needed: the 後ろ object is already implied by the verb itself. So there is some redundancy in saying 後ろを見返る, but that’s not necessarily wrong.
We should also consider the secondary meanings of 見返る and 見返す, when we’re not talking about physically looking back. 見返す has a more neutral sense of “review”, or a sense of “showing off when you win” (if you win, your opponent is, metaphorically (if not literally), behind you, so you look ‘back’ at them to humiliate them). 見返る, on the other hand, has more of a secondary sense of “being emotionally hung up over something” - this is where a secondary meaning of を itself comes into play: When you’re talking about a verb of emotion, を is often used to mark the cause of that emotion, even if it’s not a direct object. (This probably explains why 後ろ isn’t the top collocation in the chart @toc linked.)