Well maybe because she wanted to... well i don't know, revive him? i mean think about it, you can revive the Firekeeper in Firelink by bringing her soul back to her body, what stops other from doing the same, specially someone so powerful as one of the four knights.
I say Ciaran tried to revive Artorias, but was stopped... by Sif, therefore her state of deadness behind his grave (i really think that is her) but that still lingers the question, why Sif?
maybe because his master, in his last moments of sanity, ordered him not to allow such to happen? that is why he guards the tomb, to prevent anyone from traversing the abyss and ending up like Artorias AAAAAND, to prevent anyone from bringing the corrupted Abysswalker back? we'll see soon enough XD