Happy 4th july, even if I am not from states but I know what's going on there :). Anyways Cassandra made herself quite huge target for pervesion done to her (question is will Malcanthet herself do the honors, assign one incubi/succubi for it or assign one or all party members do it  >:)). I can imagine transformation to tiefling comes from corruptive influence of Malcanthet (and so does not neccesary need her personally do the defiling so long it happens in her domain and her watching the proceedings).