During eruptive flares, vector magnetograms show an increasing horizontal magnetic field and downward Lorentz force in the Sun's photosphere around the polarity-inversion line (PIL). This behavior has often been associated with the implosion conjecture and has been interpreted as the result of either momentum conservation while the eruption moves upward or of the contraction of flare loops. We characterize the physical origin of these observed behaviors by analyzing a generic 3D magnetohydrodynamics simulation of an eruptive flare. Even though the simulation was not designed to recover the magnetic field and Lorentz force properties, it is fully consistent with them, and it provides key additional information for understanding them. The area where the magnetic field increases gradually develops between current ribbons, which spread away from each other and are connected to the coronal region. This area is merely the footprint of the coronal post-flare loops, whose contraction increases their shear field component and the magnetic energy density, in line with the ideal induction equation. For simulated data, we computed the Lorentz force density map by applying the method used in observations. We obtained an increase in the downward component of the Lorentz force density around the PIL, consistent with observations. However, this significantly differs from the Lorentz force density maps that are obtained directly from the 3D magnetic field and current. These results altogether question previous interpretations that were based on the implosion conjecture and momentum conservation with the coronal mass ejection, and rather imply that the observed increases in photospheric horizontal magnetic fields result from the reconnection-driven contraction of sheared flare loops.