The Bharatiya Janata Party had formed the very first ever government in Congress dominated states of North-east India i.e. Assam, there are some salient features to be added:
BJPs acceptance about the problem of illegal immigrants gave the people of Assam a hope that BJP will do something about the same unlike Congress who did not accept the fact in their fifteen year of rule.
Their stance about keeping the reservation alive also played some role in their winning. Assam has 24 seats reserved (SC/ST) out of 126. NDA won 20 seats with BJP winning 13 seats.
Most of the Assamese are believer of Hindutva. BJP used that concept and portrayed the picture of Congress collaborating with AIUDF (Muslim Party. This had a great impact on the indigenous and Hindu people and united them. Consequently majority of Hindu voted for BJP like in Barack Valley (Cachar, Karimganj, Hailakhandi) and Upper Assam (Dibrugarh, Jorhat, Sivsagar, Tinsukia, Golaghat). Out of 65 seats in Upper Assam Congress won only 6 seats. These are the area of Assamese people who are believer of Hindutva.
The campaign management committee tactically did not allow the aborigine Muslim to be left out. It can be seen where saffron party could win in some of the Muslim majority constituency like Eastern Bilasipara and losing of Supremo Badruddin Ajmal (leader of AIUDF).
Portrayal of Sarbananda Sonowal as Chief Ministerial candidate at that time of Assam Assembly Elections also drew huge supporters. Before joining politics he was the President of the largest student body of Assam from 1992 to 1999. Before joining BJP he was a member of AGP till 2011. He is the one who took the issue of illegal immigration to the supreme court of India.
Bharatiya Janata Party always had supporter in Assam. It can be seen in the results of previous elections. In every election they could manage to win some seats. But what they lacked was a good leader who could inspire the people to come forward and support them. This was done by both Sarbananda Sonowal and Himanta Biswa Sarma. While the latter had already showed what his capability is as education and health minister in last government, the former is a trusted and respected leader of Assam. Meanwhile, congress was busy in showing nepotism and arrogance. This gave people that instant force to believe in BJP nd end the history of Congress.