It should be noted that Cixi didn't resist reform or modernization, she resisted radical political and cultural reform, which was what Kang Youwei and his reform clique was trying to push. Most history books only cite the most common ones such as changing the government into a constitutional monarchy, modernizing the examination system, introduction of capitalism, educational system reform, industrialization, but Kang and his clique were much more radical. Among the more radical reforms that Kang wanted to implement was to establish Confucianism as a state religion, end censorship of private newspapers, encourage citizens to memorialize to the emperor directly, move the capital to a more central location, wear Western clothing instead of Manchu clothing, cut the queues, and push for total equality between Manchu and Han.

Some of these reforms, such as changing dress and hairstyle, equality between Manchu and Han, and moving the capital was a direct attack against the core of the Qing political system, and this was one of the major reasons why Cixi came to oppose Guangxu's reforms. Some historians cite the Hundred Day Reform as the actual start of Guangxu's rule as Cixi "retired" into the Summer Palace, but this is actually false. According to Chinese historian Mao Haijian's study of the Hundred Days Reform in the 戊戌变法史事考, which draws on the Qing Shilu, Cixi was actually more involved with the reforms than people give her credit. During the 117 day period before the reforms, the Guangxu emperor stayed in the same palace as Cixi 69 days, paid respect to her 68 times, had breakfast with her 32 times, had dinner with her 22 times, and watched plays with her 12 times. During the reform period, Guangxu traveled to the Summer Palace 12 times, every single edict he issued included Cixi's name in the opening, and a total of 462 memorials were sent to the Summer Palace for Cixi to view, the most in a single day was 29. Cixi also had Guangxu issue three very important edicts: 1.) The emperor's tutor Weng Tonghe was dismissed from office, 2.) All new officials grade 2b and above must pay respect to her before taking office, 3.) Ronglu would be appointed Viceroy of Zhili and take charge of the capital's defenses.

From this, we can see that Cixi knew very well what was going on in the reforms, and she didn't end it until they became more and more radical and threatened her own power. Kang Youwei and his clique might have misjudged Guangxu's actual power and tried to push the reforms too hastily. They immediately ran into opposition as the court was dominated by conservatives that Cixi appointed, some of whom were related to her through marriage. Guangxu was forced to dismiss many of them from office as they tried to hinder the reforms and they all went to the Summer Palace weeping and complaining to Cixi. This was already an attack on Cixi's power, but in his haste to get the reforms implemented, Kang Youwei concluded that the only way to end opposition was the remove Cixi from the scene, so his student Tan Sitong approached Yuan Shikai, then the commander of a Westernized military unit, and asked that he aid the emperor in killing Cixi.

Yuan Shikai, being the political opportunist that he was, saw Cixi as stronger than the emperor, so he went to Cixi's confidant Ronglu and leaked the plan to him. Only then did Cixi act, returning to the Forbidden City in the middle of the night, taking power from the emperor, and ordering the arrest of the reformers.

Edit: Grammar