This was coupled with the death and retirement of influential leaders who represented lower caste interests. This led to a break in the congress's hold over these groups. This led to a burst of parties that pandered to conservative Hindus(BJP), lower castes(SP, BSP etc) /2
Oh yeah and the congress also consolidated itself around the Gandhi family(no relation). This led to a lot of very influential political leaders from the congress to splinter off and form their own parties. Since India is FPTP, they created regional bastions which made them indispensable.
This broke the congress' hegemony over indian politics. If you want a really deep dive into Indian politics, have a look at RC Guha's book India After Gandhi.