How and why do anarchists reject all forms of authority

Anarchists reject all forms of authority as they have a very optimistic view of human nature. Therefore, believe there is no need for external influence on human nature to avoid coercion. As a result, authority removes the natural balance that humans already have as the State is 'unnatural'. They also believe Authority is an offence the principles of freedom and equality. Proudhon and Bakunin believed granting authority to individuals will inevitably result in them oppressing the lower class to meet their own needs as greed takes over. This can be summarised by a famous Anarchist saying "power is corrupting and absolute power corrupts absolutely. Hence, to avoid oppression and exploitation of those not in power, all forms of authority must be removed. 

Secondly, the 'how' aspect of the question differs with different types of anarchists. Collectivist Anarchists believe in socialist ideas. The community must work together to benefit each other. Their social duty allows them to act accordingly. For this to be carried out people need to form communes and build trade unions with unpaid representatives. This created anarcho-syndicalism. Whereas, Individual anarchists are inspired by classical liberalism and the free market mechanism. Therefore, they believe in 'egoism', this is when the individual is responsible for themselves and does not need to follow rules assigned by another group or individual. Individualists want to reject authority by having a completely stateless society and individual responsibility driven by self-interest.