  CaFe$_{2}$O$_{4}$ is an $S=5/2$ antiferromagnet exhibiting two magnetic
orders which shows regions of coexistence at some temperatures. Using a Green's
function formalism, we model neutron scattering data of the spin wave
excitations in this material, ellucidating the microscopic spin Hamiltonian. In
doing so, we suggest that the low temperature A phase order
$(\uparrow\uparrow\downarrow\downarrow)$ finds its origins in the freezing of
antiphase boundaries created by thermal fluctuations in a parent B phase order
$(\uparrow\downarrow\uparrow\downarrow)$. The low temperature magnetic order
observed in CaFe$_{2}$O$_{4}$ is thus the result of a competition between the
exchange coupling along $c$, which favors the B phase, and the single-ion
anisotropy which stabilizes thermally-generated antiphase boundaries, leading
to static metastable A phase order at low temperatures.
