In the old days it was because the waterproof membrane inside the door had perished. These days doors are often steel sandwich construction and it will mean that the seal at the bottom of this has broken. Fiddly dismantling job to fix. But if you hear water sloshing about inside the door, then go round the bottom of the door with a wooden cocktail or kebab stick and poke through the drain holes. That might be all you have to do.