  The role of magnetic fields in galaxy evolution is still an unsolved question
in astrophysics. We have previously shown that magnetic fields play a crucial
role in major mergers between disc galaxies; in hydrodynamic simulations of
such mergers, the Auriga model produces compact remnants with a distinctive bar
and ring morphology. In contrast, in magnetohydrodynamic (MHD) simulations,
remnants form radially-extended discs with prominent spiral arm structure. In
this paper, we analyse a series of cosmological "zoom-in" simulations of major
mergers and identify exactly \textit{how} magnetic fields are able to alter the
outcome of the merger. We find that magnetic fields modify the transport of
angular momentum, systematically hastening the merger progress. The impact of
this altered transport depends on the orientation of the field, with a
predominantly non-azimuthal (azimuthal) orientation increasing the central
baryonic concentration (providing support against collapse). Both effects act
to suppress an otherwise existent bar-instability, which in turn leads to a
fundamentally different morphology and manifestation of feedback. We note, in
particular, that stellar feedback is substantially less influential in MHD
simulations, which allows for the later accretion of higher angular momentum
gas and the subsequent rapid radial growth of the remnant disc. A corollary of
the increased baryonic concentration in MHD simulations is that black holes are
able to grow twice as large, although this turns out to have little impact on
the remnant's development. Our results show that galaxy evolution cannot be
modelled correctly without including magnetic fields.
