Weak ferromagnetism and spin reorientation in antiferroelectric <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>BiCrO</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:math>

${\mathrm{BiCrO}}_{3}$ is an antiferroelectric perovskite known to exhibit unconventional spin reorientation transition between antiferromagnetic structures, accompanied by a large jump in weak ferromagnetism. Using combination of neutron powder diffraction, magnetometry, and symmetry analysis, we confirm the dominant G-type order below ${T}_{\mathrm{N}}=111$ K identify magnetic phase with spontaneous rotation ${\mathrm{Cr}}^{3+}$ moments from $b$ axis particular direction $ac$ plane. We demonstrate role displacements produced ${\mathrm{Bi}}^{3+}$ lone-pair electrons octahedral rotations establishing canting via antisymmetric Dzyaloshinskii-Moriya interaction. This mechanism results ferromagnetism above spin-reorientation explains dramatic increase net magnetization on cooling.