Based on dynamical calculations, we theoretically study the nucleation of stripe domains in single-crystal Co bars. Three different stripe domain structures at remanence are obtained in micromagnetic simulations depending on different field histories. We show that the nucleation of all three stripe domains are related to soft mode instabilities. When the field is along the long axis of the bar, the remanent stripe domain structure is shown to be generated by a standing-wave mode, that has the same spatial structure as the stripes at remanence and goes soft at a second-order phase transition where the stripe domains emerge. For the other two directions of the field, we find that the symmetry of soft modes is consistent with the change in symmetry of the ground state but in these cases the phase transition is first order.