By several accounts, Ace had a unique timeline which was heavily interfered with on multiple occasions, (PROSE: Interference, At Childhood's End, Sometime Never..., AUDIO: Assassins) leading to it being fractured across multiple realities. (PROSE: At Childhood's End, AUDIO: Signs and Wonders)
Fenric was the initial augmenter of Ace's timeline, positioning her into a Wolf of Fenric. In a process similar to but more intentional than the Eighth Doctor's alterations to Sam Jones's timeline, the Seventh Doctor undid Fenric's manipulations using temporal equations. As these equations were done instinctively, Ace's new destiny didn't go quite as the Doctor expected. (PROSE: Interference)