  By analysing a sample of galaxies selected from the HI Parkes All Sky Survey
(HIPASS) to contain more than 2.5 times their expected HI content based on
their optical properties, we investigate what drives these HI eXtreme (HIX)
galaxies to be so HI-rich. We model the HI kinematics with the Tilted Ring
Fitting Code TiRiFiC and compare the observed HIX galaxies to a control sample
of galaxies from HIPASS as well as simulated galaxies built with the
semi-analytic model Dark Sage. We find that (1) HI discs in HIX galaxies are
more likely to be warped and more likely to host HI arms and tails than in the
control galaxies, (2) the average HI and average stellar column density of HIX
galaxies is comparable to the control sample, (3) HIX galaxies have higher HI
and baryonic specific angular momenta than control galaxies, (4) most HIX
galaxies live in higher-spin haloes than most control galaxies. These results
suggest that HIX galaxies are HI-rich because they can support more HI against
gravitational instability due to their high specific angular momentum. The
majority of the HIX galaxies inherits their high specific angular momentum from
their halo. The HI content of HIX galaxies might be further increased by
gas-rich minor mergers. This paper is based on data obtained with the Australia
Telescope Compact Array (ATCA) through the large program C 2705.
