How they were made Untouchables
The Manusmriti known as laws translated "Laws of Manu" is a foundational work of Brahmin law, written c.200 in India. It is one of the eighteen Smritis of the Dharma Sastra (or "laws of righteous conduct"). The Smritis contain laws, rules and codes of conduct to be applied by individuals, communities and the nations.
Manusmriti was written after the collapse of Buddhism and was written as a new Constitution to grow Brahmanism and with an intention to root out those that opposed Brahmanism. In order to prevail and uphold Brahmanism the Caste system was introduced and various stories were written and new stories were added / inserted at that time to the then existing known Historic Stories like Mahabharata and Ramayana to give a confusing picture as if Caste System existed in India since civilization existed in India. Manu's rules made provision to ex-communicate people that did not adopt Brahmanism and hence the caste system. The people (Great Ruling Dynasties of ancient and Middle ages in India that opposed Brahmanism) gradually became Untouchables over a period of several Generations.
During the time of Manu (2nd Century AD) the descendants of the rulers at the actual time of Mahabharata that became poor during Manu's time were branded by Manu as Bastards. The Kings that did not adopt Brahmanism during Manu's time were branded as 'Degraded Kings - Vratya Kshatriyas' and Manu gave rules as how these Kings can become noble Kings by adopting Brahmanism and by supporting Brahmins. The Kings and his descendants that did not adopt and support Brahmanism were left in the Vratya Kshatriyas category and were ex-communicated once they lost power. These were the people that gradually became Untouchables. To hide this historic facts Brahmins kept modifying History and wrote stories and cleverly covered this under the disguise of Religion. The British also aided the Brahmins to do this. So unfortunately till now the true History of Caste and Untouchables were hidden.
For more details please review
The origin of Brahmanism, Caste and Riddles in Hinduism
Who are the Paraiyars
Who are the Brahmins and who are the Aryans