This interesting tradition may have come from an innocent mistake...
One Chinese New Year's day (the exact year is not known now), a family attached their 福 (fu) upside down as a careless mistake. On the first day of Chinese New Year, their first guest came to visit, and saw the upside-down 福, and kindly shouted to them: "你们的福倒了!" ('Your fu is upside down!').
倒 (dao /daoww/) means 'to invert', but also means 'to pour out'. So "你们的福倒了" could be understood as ‘Your fu (blessing) has been poured out’. People liked the alternative meaning so much that they started fixing their fu decorations upside down to "invoke" a 'pouring out' of blessings.