Why is my character moving backwards?

Solution 1:

In the latest set that was added for free (you still have to buy them in-game), one of the cards is "Lost Child".

Move backwards while this card is held. Cannot Norma while holding this card. This card is discarded upon KO or use.

Even if you didn't buy one yourself, someone else could have and everyone's deck is mixed for a game.

Lost Child is a gift type card, and like other gift type cards, an icon floats around your head to indicate you're holding it.

Solution 2:

This is a unique field event called Backtrack defined in the wiki as Every 8 chapters, reverses the direction of movement for one chapter.