How was the transformation of classes into masses possible?

In Arendt's The Origin of Totalitarianism, the grouping of people can seamlessly transition from a "class" structure to a "mass of humanity." Classes, she explains, are the natural social divisions that are inherently present in large groups of people. Because of their similarities, people of a certain class feel a sense of unity and even pride of being in a certain class.
Over time, however, especially in the more recent industrial ages, these classes begin to break down. The community bonds are harder to forge because of the larger size of societies, and individuals act in their own best interests more than in the interests of society or the class at large. In the presence of a totalitarian leader, such as Hitler or Mussolini, this mass of individuals can easily be swayed and influenced, as they are all concerned with self-preservation and self-interest. They will act as a group to save themselves—for instance, people in Nazi Germany were quick to distance themselves from Jewish individuals. Even though they are acting as individuals, they are doing so as a whole population, and a dictator can use this to their advantage to leverage a society for their personal gain. In this way, the classes can become the masses.

According to Arendt in The Origins of Totalitarianism, people in modern society become alienated from each other. They experience anomie, which is the breakdown of traditional social bonds and traditional morality. People begin to freely float as random individuals rather than experience themselves primarily as members of a group. This makes the transformation from class to mass possible.
Arendt calls the traditional groups people once identified with "classes." Classes had a strong sense of collective identity, with shared moral values. They also had a shared sense of purpose.
When totalitarian dictators come along, they exploit anomie. They pull together a large number of disparate individuals, which Arendt calls a mass rather than a class. Theses masses are easily manipulated because they don't have a strong sense of group identity. What the mass desires most of all is a sense of belonging and faith that a strong leader is in charge. A mass isn't that concerned with the content of what they are asked to believe in. The dictator is thus able to mold the mass any way he wants. Arendt argues that the shallowness of conviction in a mass movement is why so many Germans were able to abandon Nazism rapidly and completely after the war.