Anything from the bushings on the steering gear to the bushings in the steering spindles, to bent drag links to a slight flex in the frame to wear in the axle pivot can cause the issues that you are seeing. When they went from the tie rod type steering with the single drag link to the dual drag link steering toe out issues have been a pain to correct, and a bigger issue for the mower owner. It allows the mower to make have a shorter turning radius but fighting that alignment issue is ongoing.