This revised macro should work for you...
Code:
Sub SplitSelectedCellsDown()
  Dim Cell As Range, Sentences() As String
  For Each Cell In Selection
    Sentences = Split(Cell.Value & vbLf, vbLf)
    Cell.Offset(1).Resize(UBound(Sentences)).Insert xlShiftDown
    Cell.Offset(1).Resize(UBound(Sentences)) = Application.Transpose(Sentences)
  Next
End Sub