When running pdftocairo with -paperw and -paperh arguments, these dimensioners are swapped when a PDF in landscape is given as input file. Thus, width becomes height and height becomes width.
This behavior seems strange, as -W and -H arguments passed when cropping a PDF in landscape mode are not swapped.
I suggest consistent behavior where -paperw and -paperh denote the width and height of the output file, respectively (and not vice versa).
This change was made in poppler-0.38.