Because Mi Fang and Guan Yu have always been at odds.

Guan Yu has always looked down on Mi Fang. Although Mi Fang is related to Liu Bei, we know that Guan Yu has always been arrogant. There are few people in the world who are truly convinced by him. Even Sun Quan sent an envoy to say goodbye. The two became in-laws, but both were scolded by Guan Yu. This shows how arrogant Guan Yu was, and it is not surprising that he despised Mi Fang.

Mi Fang and Fu Shiren had problems with food and grass many times. Each time, Guan Yu severely reprimanded them and said, "They should still be dealt with." It means that I will take care of you after I come back from the war. This made Mi Fang fearful, fearing that Guan Yu would come to settle the score with him, which prompted Mi Fang to rebel.

Although Mi Fang knew Guan Yu's strength, he was not so confident in Guan Yu under the joint campaign of Sun Quan and Cao Cao, and was very worried that Guan Yu would fail. But once Guan Yu failed, he would be afraid that Liu Bei would be punished by Liu Bei, and even thought of being killed by Liu Bei. Even as Liu Bei's adopted son, he could not escape the fate of being killed in the end, and Mi Fang's worries were not unreasonable.

Mi Fang rebelled and surrendered to Sun Quan:

At the critical moment when Lu Meng sent troops to attack Jingzhou, Mi Fang and Fu Shiren actually chose Kaicheng to surrender to Sun Quan. Their naked betrayal really cost Guan Yu A heavy blow made him homeless, and he died tragically under Lu Meng's sword.