Dementorerna hade länge övergivit ministeriet.

Dementorerna hade redan (öppet) varit lojala mot Voldemort i två år vid den tid som slaget vid Hogwarts ägde rum. De gick direkt till Voldemort efter att han återvände offentligt med ministeriet.

"It is great regret that I must confirm that the wizard styling himself Lord - well, you know who I mean - is alive and among us again," said Fudge, looking tired and flustered as he addressed reporters. "It is with almost equal regret that we must report the mass revolt of the Dementors of Azkaban, who have shown themselves adverse to continuing in the Ministry's employ. We believe the Dementors are currently taking direction from Lord - Thingy."
(Order of the Phoenix, Chapter 38, The Second War Begins).

Voldemort hade planerat detta drag sedan han återvände i Goblet of Fire .

...when Azkaban is broken open, the Lestranges will be honoured beyond their wildest dreams. The Dementors will join us...they are our natural allies...
(Goblet of Fire, Chapter 33, The Death Eaters).

Även Dumbledore förutspådde att dementorerna hade mycket mer gemensamt med Death Eaters än med ministeriet.

"The first and most essential step is to remove Azkaban from the control of the Dementors -"
"Preposterous!" shouted Fudge again. "Remove the Dementors! I'd be kicked out of office for suggesting it! Half of us only feel safe in our beds at night because he know the Dementors are standing guard at Azkaban!"
"The rest of us sleep less soundly in our beds, Cornelius, knowing that you have put Lord Voldemort's most dangerous supporters in the care of creatures who will join him the instant he asks him!" said Dumbledore. "They will not remain loyal to you, Fudge! Voldemort can offer them much more scope for their powers and their pleasures than you can!"
(Goblet of Fire, Chapter 36, The Parting of the Ways).

Eftersom Voldemort gjorde det möjligt för Dementors fria regering att dränera lyckan och hoppet ut av alla (Muggles och trollkarlar, brottslingar och oskyldiga), kom Dementorsna med glädje till honom när hans återvändande öppet förklarades av ministeriet.