The driving force behind the Singh Sabha Movement was the desire to answer the question "what is a Sikh?"
At the time of the Singh Sabha Movement, Gurdwaras were controlled by Udasis and Mahants and many Hindu practices had started in Gurdwaras including idol worship and caste discrimination.
Another problem facing the Sikh community was the threat of conversions. Many Sikhs had converted to Islam, others to Christianity. Later on, some even began reverting to Hinduism through the ceremony of "shuddhi" where their hair were publicly shaved and they repeated Vedic mantras.
There were two Singh Sabha groups: Amritsar and Lahore. The Amritsar group were Sanatanists who thought Sikhs were a branch of Hindus. They believed in caste and a number of Hindu ceremonies. The Lahore group (which 'won') believed Sikhs were a distinct faith and had leaders such as Giani Gurdit Singh and Bhai Gurmukh Singh. They wrote many articles and had many debates and showed Sikhs were indeed distinct.
The Akali movement is a linked movement which was intiated to re-take Sikh Gurdwaras into Panthic control.
Although it has almost become fashionable to bash the Singh Sabha movement these days, the fact is that it was essential in preserving the real message of Sikhi and without it, we would have been in serious trouble.