Os dementadores há muito haviam abandonado o Ministério.

Os Dementadores já eram (abertamente) leais a Voldemort por dois anos no momento em que a Batalha de Hogwarts aconteceu. Eles se juntaram a Voldemort logo após seu retorno ter sido publicamente reconhecido pelo Ministério.

"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 planejou esse movimento desde seu retorno em Cálice de Fogo .

...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).}

Até mesmo Dumbledore previu que os dementadores tinham muito mais em comum com os Comensais da Morte do que com o Ministério.

"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).}

Desde que Voldemort permitiu que o reinado livre dos Dementadores drenasse a felicidade e a esperança de todos (trouxas e bruxos, criminosos e inocentes), os dementadores se juntaram a ele alegremente no momento em que seu retorno foi abertamente declarado pelo Ministério.