The reason for India's score falling in the report is the deployment of large number of military forces in the wake of the removal of Article 370 from Jammu and Kashmir, the detention of local leaders, the exclusion of large Muslim population from the implementation of NRC in Assam, the Citizenship Amendment Act. Displeasure of large Muslim population, student agitation in JNU, police action in Jamia Millia Islamia University and Shaheen Bag, The ongoing protests in Nagaland.
The Democracy Index is an index compiled by the Economist Intelligence Unit (EIU), a UK-based company. It intends to measure the state of democracy in 167 countries, of which 166 are sovereign states and 164 are UN member states. You can download the 2019 report here https://www.eiu.com/topic/democracy-index