  We use the EAGLE cosmological hydrodynamic simulation suite to study the
specific angular momentum of galaxies, $j$, with the aims of (i) investigating
the physical causes behind the wide range of $j$ at fixed mass and (ii)
examining whether simple, theoretical models can explain the seemingly complex
and non-linear nature of the evolution of $j$. We find that $j$ of the stars,
$j_{\rm stars}$, and baryons, $j_{\rm bar}$, are strongly correlated with
stellar and baryon mass, respectively, with the scatter being highly correlated
with morphological proxies such as gas fraction, stellar concentration, (u-r)
intrinsic colour, stellar age and the ratio of circular velocity to velocity
dispersion. We compare with available observations at $z=0$ and find excellent
agreement. We find that $j_{\rm bar}$ follows the theoretical expectation of an
isothermal collapsing halo under conservation of specific angular momentum to
within $\approx 50$%, while the subsample of rotation-supported galaxies are
equally well described by a simple model in which the disk angular momentum is
just enough to maintain marginally stable disks. We extracted evolutionary
tracks of the stellar spin parameter of EAGLE galaxies and found that the fate
of their $j_{\rm stars}$ at $z=0$ depends sensitively on their star formation
and merger histories. From these tracks, we identified two distinct physical
channels behind low $j_{\rm stars}$ galaxies at $z=0$: (i) galaxy mergers, and
(ii) early star formation quenching. The latter can produce galaxies with low
$j_{\rm stars}$ and early-type morphologies even in the absence of mergers.
