Pronounced spin precessions are observed in a geometry with negligible canting of the magnetization in ferromagnetic La0.67Ca0.33MnO3 thin films using the time-resolved magneto-optical Kerr effect. The precession amplitude monotonically decreases with increasing field, indicating that the coherent spin rotation may be triggered by a transient exchange field and not by demagnetization and/or anisotropy field modulation. We attribute the transient exchange field to emergent antiferromagnetic interactions due to charge transfer and modification of the kinetic energy of eg electrons under optical excitation.