Why did Dumbledore leave Hogwarts in Harry Potter and the Order of the Phoenix?

because well first of all, he keeps making speeches that "you-know-who" is back and the ministry is in denial about that. then he keeps harry from being expelled from hogwarts at the beginning (because harry and dudley were attacked by dementors and he had to use magic to save himself but the ministry doesn't believe there were dementors so they say that he was just using magic for the fun of it in front of a muggle and when you are underage you can't do magic so he was going to be expelled from hogwarts) and so dumbledore pi$$es him off there. and finally, Fudge (the minister of magic) thinks that dumbledore wants his job and is building a "dumbledore's army" to take over the ministry. so in fear he makes dolores umbridge the new headmaster, dumbledore is demoted from Chief Warlock of the Wizengamot, voted out of the Chairmanship of the International Confederation of Wizards, and is almost stripped of his Order of Merlin First Class. but dumbledore is funny and says that he doesn't care as long as they don't take him off the chocolate frog cards.

p.s. there is a dumbledore's army, but it is a group of students that harry is teaching defense against dark arts to, they only named it dumbledore's army to make fun of the ministry, which backfires later on.

Most of the Wizarding World was against Dumbledore because of their refusal to believe that Voldemort was back, which Dumbledore kept on stating explicitly. He was already becoming downgraded, no longer the Chief Warlock of the Wizengamot or the Supreme Mugwump of the International Confederation of Wizards, and the entire Ministry was against him. However, he didn't care, as long as they didn't take him off the Chocolate Frog Cards. He could not rebuke Fudge's decision to make Umbridge the Hogwarts High Inquisitor.

But he specifically left Hogwarts when Dumbledore's Army was turned into Umbridge by Marietta Edgecombe. Not wanting the students to get in trouble, Dumbledore assured Umbridge/Fudge that he had instructed them to form the group to fight against the Ministry. However, before they could take him to Azkaban, he overpowered his captors and escaped to work for the Order.

Because the ministry was taking over Hogwarts, he was about to be kicked out of the school and he wanted to leave on his terms instead of being taken to Azkaban for conspiring against the ministry. Also to take the blame off harry.

The ministry fired him and made Umbridge headmistress because they believed he was behind Dumbledore's Army.

i do not think that dumbledore should leave hogwarts

he was trying to find voldemort's horcruxes.finally,he finds one-the ring.