Most birds will stay on the wing on their journey home from a race or training but there are circumstances that may result in the bird landing on its journey home. The most obvious one I would say would be a BOP strike were most birds will dive for cover in trees, hedge rows, buildings etc and will usually sit long enough before returning to the sky on their journey. Others already mentioned landing for water on a very warm day, running out of daylight hours especially on the longer flights and having to rest up over night before setting off again the next morning. Unfortunate reasons are birds not trained well enough and are not fit to complete their journey and of course the various obstacles like electricity pylon wires, land fencing and rogue gunclub members out on a shoot for the day. :(