  We use spin-density-functional theory to study recently reported hysteretic
magnetoresistance \rho_{xx} spikes in Mn-based 2D electron gases
[Jaroszy\'{n}ski et al. Phys. Rev. Lett. (2002)]. We find hysteresis loops in
our calculated Landau fan diagrams and total energies signaling
quantum-Hall-ferromagnet phase transitions. Spin-dependent exchange-correlation
effects are crucial to stabilize the relevant magnetic phases arising from
distinct symmetry-broken excited- and ground-state solutions of the Kohn-Sham
equations. Besides hysteretic spikes in \rho_{xx}, we predict hysteretic dips
in the Hall resistance \rho_{xy}. Our theory, without domain walls,
satisfactorily explains the recent data.
