mistfist
Protector of Moe
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To be fair, Tart being Walpurgisnacht hasn't been 100% proven yet in Tart Magica, but the amount of evidence pointing towards this being true makes it practically guaranteed.Tart Magica implies that Walpurgisnacht is the witch that Joan of Arc/Tart became, and Madoka/Homura's story resonates strongly with her own story (referring to the specific events of Tart Magica); furthermore, once Homura made her Wish in the first timeline, and then looped, Madoka's potential started to grow enormously, such that Madoka herself (and Homura, by extension) became so historically significant that Walpurgisnacht was effectively drawn to them like a lightning rod.
We don't know why Walpurgisnacht is drawn to Madoka and Homura outside of the massive potential, only that she is. It could be that their story of tragedy resonates with what happened to Tart and Riz, or maybe Walpurgisnacht just enjoys creating maximum amounts of despair, but until the final two Tart volumes come out (September and November this year), we can't know for sure.