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Metal-insulator transitions (MITs) in strongly-correlated materials result from the interplay of many degrees of freedom, such as electronic, magnetic or structural. Untangling these contributions has remained a longstanding problem in condensed matter physics. In this work, we have investigated V₂O₃, an archetypal strongly-correlated material with a MIT where electronic, structural and magnetic phase transitions occur simultaneously. By performing magneto-resistance (MR) measurements across the MIT we acted on the magnetic degree of freedom and revealed an anomalous behavior: the MR crosses over to negative values and seemly diverges as the MIT takes place. To gain physical insight, we study the antiferromagnetic MIT in a Hubbard model in the presence of a magnetic field by high precision dynamical mean-field theory calculations. We find the model results accurately reproduce the unusual experimental behavior. Furthermore, they reveal a simple mechanism where the magnetic field impedes the antiferromagnetic ordering of one sublattice, thus preventing the opening of the gap. Our study provides strong evidence that the origin of the MIT in V₂O₃ is the opening of an antiferromagnetic gap.