Grapefruit Fermentation Super Speed

[AdamBrandy]

I have never seen fruit juice ferment so fast as this grapefruit must! On all my ferments I use Actiflore F-33 yeast by Laffort. Usually - no matter if I ferment my squeezed apple must, date syrup wash or bitter orange - they take two weeks to finish fermentation. This time I squeezed about 15 gal of grapefruit juice (red and white), added two kg of brown sugar, pitched in 10 gr of nutrients and 20 gr yeast - and after four days of vigorous ferment the airlock stopped bubbling. I checked the hydrometer and it seems to have dried out. No sweet on the tung neither. How is this possible? Anyone has an explanation?
*It can't be temperature because it was between 14-21 degrees celsius all the time.